From 1b593e1ea4d2af0f6444d9a7788d5d99abd6fde5 Mon Sep 17 00:00:00 2001
From: Vincent Ambo <Vincent Ambo>
Date: Sat, 11 Jan 2020 23:36:56 +0000
Subject: Squashed 'third_party/git/' content from commit cb71568594

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+git-fast-import(1)
+==================
+
+NAME
+----
+git-fast-import - Backend for fast Git data importers
+
+
+SYNOPSIS
+--------
+[verse]
+frontend | 'git fast-import' [<options>]
+
+DESCRIPTION
+-----------
+This program is usually not what the end user wants to run directly.
+Most end users want to use one of the existing frontend programs,
+which parses a specific type of foreign source and feeds the contents
+stored there to 'git fast-import'.
+
+fast-import reads a mixed command/data stream from standard input and
+writes one or more packfiles directly into the current repository.
+When EOF is received on standard input, fast import writes out
+updated branch and tag refs, fully updating the current repository
+with the newly imported data.
+
+The fast-import backend itself can import into an empty repository (one that
+has already been initialized by 'git init') or incrementally
+update an existing populated repository.  Whether or not incremental
+imports are supported from a particular foreign source depends on
+the frontend program in use.
+
+
+OPTIONS
+-------
+
+--force::
+	Force updating modified existing branches, even if doing
+	so would cause commits to be lost (as the new commit does
+	not contain the old commit).
+
+--quiet::
+	Disable the output shown by --stats, making fast-import usually
+	be silent when it is successful.  However, if the import stream
+	has directives intended to show user output (e.g. `progress`
+	directives), the corresponding messages will still be shown.
+
+--stats::
+	Display some basic statistics about the objects fast-import has
+	created, the packfiles they were stored into, and the
+	memory used by fast-import during this run.  Showing this output
+	is currently the default, but can be disabled with --quiet.
+
+Options for Frontends
+~~~~~~~~~~~~~~~~~~~~~
+
+--cat-blob-fd=<fd>::
+	Write responses to `get-mark`, `cat-blob`, and `ls` queries to the
+	file descriptor <fd> instead of `stdout`.  Allows `progress`
+	output intended for the end-user to be separated from other
+	output.
+
+--date-format=<fmt>::
+	Specify the type of dates the frontend will supply to
+	fast-import within `author`, `committer` and `tagger` commands.
+	See ``Date Formats'' below for details about which formats
+	are supported, and their syntax.
+
+--done::
+	Terminate with error if there is no `done` command at the end of
+	the stream.  This option might be useful for detecting errors
+	that cause the frontend to terminate before it has started to
+	write a stream.
+
+Locations of Marks Files
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+--export-marks=<file>::
+	Dumps the internal marks table to <file> when complete.
+	Marks are written one per line as `:markid SHA-1`.
+	Frontends can use this file to validate imports after they
+	have been completed, or to save the marks table across
+	incremental runs.  As <file> is only opened and truncated
+	at checkpoint (or completion) the same path can also be
+	safely given to --import-marks.
+
+--import-marks=<file>::
+	Before processing any input, load the marks specified in
+	<file>.  The input file must exist, must be readable, and
+	must use the same format as produced by --export-marks.
+	Multiple options may be supplied to import more than one
+	set of marks.  If a mark is defined to different values,
+	the last file wins.
+
+--import-marks-if-exists=<file>::
+	Like --import-marks but instead of erroring out, silently
+	skips the file if it does not exist.
+
+--[no-]relative-marks::
+	After specifying --relative-marks the paths specified
+	with --import-marks= and --export-marks= are relative
+	to an internal directory in the current repository.
+	In git-fast-import this means that the paths are relative
+	to the .git/info/fast-import directory. However, other
+	importers may use a different location.
++
+Relative and non-relative marks may be combined by interweaving
+--(no-)-relative-marks with the --(import|export)-marks= options.
+
+Performance and Compression Tuning
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+--active-branches=<n>::
+	Maximum number of branches to maintain active at once.
+	See ``Memory Utilization'' below for details.  Default is 5.
+
+--big-file-threshold=<n>::
+	Maximum size of a blob that fast-import will attempt to
+	create a delta for, expressed in bytes.  The default is 512m
+	(512 MiB).  Some importers may wish to lower this on systems
+	with constrained memory.
+
+--depth=<n>::
+	Maximum delta depth, for blob and tree deltification.
+	Default is 50.
+
+--export-pack-edges=<file>::
+	After creating a packfile, print a line of data to
+	<file> listing the filename of the packfile and the last
+	commit on each branch that was written to that packfile.
+	This information may be useful after importing projects
+	whose total object set exceeds the 4 GiB packfile limit,
+	as these commits can be used as edge points during calls
+	to 'git pack-objects'.
+
+--max-pack-size=<n>::
+	Maximum size of each output packfile.
+	The default is unlimited.
+
+fastimport.unpackLimit::
+	See linkgit:git-config[1]
+
+PERFORMANCE
+-----------
+The design of fast-import allows it to import large projects in a minimum
+amount of memory usage and processing time.  Assuming the frontend
+is able to keep up with fast-import and feed it a constant stream of data,
+import times for projects holding 10+ years of history and containing
+100,000+ individual commits are generally completed in just 1-2
+hours on quite modest (~$2,000 USD) hardware.
+
+Most bottlenecks appear to be in foreign source data access (the
+source just cannot extract revisions fast enough) or disk IO (fast-import
+writes as fast as the disk will take the data).  Imports will run
+faster if the source data is stored on a different drive than the
+destination Git repository (due to less IO contention).
+
+
+DEVELOPMENT COST
+----------------
+A typical frontend for fast-import tends to weigh in at approximately 200
+lines of Perl/Python/Ruby code.  Most developers have been able to
+create working importers in just a couple of hours, even though it
+is their first exposure to fast-import, and sometimes even to Git.  This is
+an ideal situation, given that most conversion tools are throw-away
+(use once, and never look back).
+
+
+PARALLEL OPERATION
+------------------
+Like 'git push' or 'git fetch', imports handled by fast-import are safe to
+run alongside parallel `git repack -a -d` or `git gc` invocations,
+or any other Git operation (including 'git prune', as loose objects
+are never used by fast-import).
+
+fast-import does not lock the branch or tag refs it is actively importing.
+After the import, during its ref update phase, fast-import tests each
+existing branch ref to verify the update will be a fast-forward
+update (the commit stored in the ref is contained in the new
+history of the commit to be written).  If the update is not a
+fast-forward update, fast-import will skip updating that ref and instead
+prints a warning message.  fast-import will always attempt to update all
+branch refs, and does not stop on the first failure.
+
+Branch updates can be forced with --force, but it's recommended that
+this only be used on an otherwise quiet repository.  Using --force
+is not necessary for an initial import into an empty repository.
+
+
+TECHNICAL DISCUSSION
+--------------------
+fast-import tracks a set of branches in memory.  Any branch can be created
+or modified at any point during the import process by sending a
+`commit` command on the input stream.  This design allows a frontend
+program to process an unlimited number of branches simultaneously,
+generating commits in the order they are available from the source
+data.  It also simplifies the frontend programs considerably.
+
+fast-import does not use or alter the current working directory, or any
+file within it.  (It does however update the current Git repository,
+as referenced by `GIT_DIR`.)  Therefore an import frontend may use
+the working directory for its own purposes, such as extracting file
+revisions from the foreign source.  This ignorance of the working
+directory also allows fast-import to run very quickly, as it does not
+need to perform any costly file update operations when switching
+between branches.
+
+INPUT FORMAT
+------------
+With the exception of raw file data (which Git does not interpret)
+the fast-import input format is text (ASCII) based.  This text based
+format simplifies development and debugging of frontend programs,
+especially when a higher level language such as Perl, Python or
+Ruby is being used.
+
+fast-import is very strict about its input.  Where we say SP below we mean
+*exactly* one space.  Likewise LF means one (and only one) linefeed
+and HT one (and only one) horizontal tab.
+Supplying additional whitespace characters will cause unexpected
+results, such as branch names or file names with leading or trailing
+spaces in their name, or early termination of fast-import when it encounters
+unexpected input.
+
+Stream Comments
+~~~~~~~~~~~~~~~
+To aid in debugging frontends fast-import ignores any line that
+begins with `#` (ASCII pound/hash) up to and including the line
+ending `LF`.  A comment line may contain any sequence of bytes
+that does not contain an LF and therefore may be used to include
+any detailed debugging information that might be specific to the
+frontend and useful when inspecting a fast-import data stream.
+
+Date Formats
+~~~~~~~~~~~~
+The following date formats are supported.  A frontend should select
+the format it will use for this import by passing the format name
+in the --date-format=<fmt> command-line option.
+
+`raw`::
+	This is the Git native format and is `<time> SP <offutc>`.
+	It is also fast-import's default format, if --date-format was
+	not specified.
++
+The time of the event is specified by `<time>` as the number of
+seconds since the UNIX epoch (midnight, Jan 1, 1970, UTC) and is
+written as an ASCII decimal integer.
++
+The local offset is specified by `<offutc>` as a positive or negative
+offset from UTC.  For example EST (which is 5 hours behind UTC)
+would be expressed in `<tz>` by ``-0500'' while UTC is ``+0000''.
+The local offset does not affect `<time>`; it is used only as an
+advisement to help formatting routines display the timestamp.
++
+If the local offset is not available in the source material, use
+``+0000'', or the most common local offset.  For example many
+organizations have a CVS repository which has only ever been accessed
+by users who are located in the same location and time zone.  In this
+case a reasonable offset from UTC could be assumed.
++
+Unlike the `rfc2822` format, this format is very strict.  Any
+variation in formatting will cause fast-import to reject the value.
+
+`rfc2822`::
+	This is the standard email format as described by RFC 2822.
++
+An example value is ``Tue Feb 6 11:22:18 2007 -0500''.  The Git
+parser is accurate, but a little on the lenient side.  It is the
+same parser used by 'git am' when applying patches
+received from email.
++
+Some malformed strings may be accepted as valid dates.  In some of
+these cases Git will still be able to obtain the correct date from
+the malformed string.  There are also some types of malformed
+strings which Git will parse wrong, and yet consider valid.
+Seriously malformed strings will be rejected.
++
+Unlike the `raw` format above, the time zone/UTC offset information
+contained in an RFC 2822 date string is used to adjust the date
+value to UTC prior to storage.  Therefore it is important that
+this information be as accurate as possible.
++
+If the source material uses RFC 2822 style dates,
+the frontend should let fast-import handle the parsing and conversion
+(rather than attempting to do it itself) as the Git parser has
+been well tested in the wild.
++
+Frontends should prefer the `raw` format if the source material
+already uses UNIX-epoch format, can be coaxed to give dates in that
+format, or its format is easily convertible to it, as there is no
+ambiguity in parsing.
+
+`now`::
+	Always use the current time and time zone.  The literal
+	`now` must always be supplied for `<when>`.
++
+This is a toy format.  The current time and time zone of this system
+is always copied into the identity string at the time it is being
+created by fast-import.  There is no way to specify a different time or
+time zone.
++
+This particular format is supplied as it's short to implement and
+may be useful to a process that wants to create a new commit
+right now, without needing to use a working directory or
+'git update-index'.
++
+If separate `author` and `committer` commands are used in a `commit`
+the timestamps may not match, as the system clock will be polled
+twice (once for each command).  The only way to ensure that both
+author and committer identity information has the same timestamp
+is to omit `author` (thus copying from `committer`) or to use a
+date format other than `now`.
+
+Commands
+~~~~~~~~
+fast-import accepts several commands to update the current repository
+and control the current import process.  More detailed discussion
+(with examples) of each command follows later.
+
+`commit`::
+	Creates a new branch or updates an existing branch by
+	creating a new commit and updating the branch to point at
+	the newly created commit.
+
+`tag`::
+	Creates an annotated tag object from an existing commit or
+	branch.  Lightweight tags are not supported by this command,
+	as they are not recommended for recording meaningful points
+	in time.
+
+`reset`::
+	Reset an existing branch (or a new branch) to a specific
+	revision.  This command must be used to change a branch to
+	a specific revision without making a commit on it.
+
+`blob`::
+	Convert raw file data into a blob, for future use in a
+	`commit` command.  This command is optional and is not
+	needed to perform an import.
+
+`checkpoint`::
+	Forces fast-import to close the current packfile, generate its
+	unique SHA-1 checksum and index, and start a new packfile.
+	This command is optional and is not needed to perform
+	an import.
+
+`progress`::
+	Causes fast-import to echo the entire line to its own
+	standard output.  This command is optional and is not needed
+	to perform an import.
+
+`done`::
+	Marks the end of the stream. This command is optional
+	unless the `done` feature was requested using the
+	`--done` command-line option or `feature done` command.
+
+`get-mark`::
+	Causes fast-import to print the SHA-1 corresponding to a mark
+	to the file descriptor set with `--cat-blob-fd`, or `stdout` if
+	unspecified.
+
+`cat-blob`::
+	Causes fast-import to print a blob in 'cat-file --batch'
+	format to the file descriptor set with `--cat-blob-fd` or
+	`stdout` if unspecified.
+
+`ls`::
+	Causes fast-import to print a line describing a directory
+	entry in 'ls-tree' format to the file descriptor set with
+	`--cat-blob-fd` or `stdout` if unspecified.
+
+`feature`::
+	Enable the specified feature. This requires that fast-import
+	supports the specified feature, and aborts if it does not.
+
+`option`::
+	Specify any of the options listed under OPTIONS that do not
+	change stream semantic to suit the frontend's needs. This
+	command is optional and is not needed to perform an import.
+
+`commit`
+~~~~~~~~
+Create or update a branch with a new commit, recording one logical
+change to the project.
+
+....
+	'commit' SP <ref> LF
+	mark?
+	original-oid?
+	('author' (SP <name>)? SP LT <email> GT SP <when> LF)?
+	'committer' (SP <name>)? SP LT <email> GT SP <when> LF
+	('encoding' SP <encoding>)?
+	data
+	('from' SP <commit-ish> LF)?
+	('merge' SP <commit-ish> LF)?
+	(filemodify | filedelete | filecopy | filerename | filedeleteall | notemodify)*
+	LF?
+....
+
+where `<ref>` is the name of the branch to make the commit on.
+Typically branch names are prefixed with `refs/heads/` in
+Git, so importing the CVS branch symbol `RELENG-1_0` would use
+`refs/heads/RELENG-1_0` for the value of `<ref>`.  The value of
+`<ref>` must be a valid refname in Git.  As `LF` is not valid in
+a Git refname, no quoting or escaping syntax is supported here.
+
+A `mark` command may optionally appear, requesting fast-import to save a
+reference to the newly created commit for future use by the frontend
+(see below for format).  It is very common for frontends to mark
+every commit they create, thereby allowing future branch creation
+from any imported commit.
+
+The `data` command following `committer` must supply the commit
+message (see below for `data` command syntax).  To import an empty
+commit message use a 0 length data.  Commit messages are free-form
+and are not interpreted by Git.  Currently they must be encoded in
+UTF-8, as fast-import does not permit other encodings to be specified.
+
+Zero or more `filemodify`, `filedelete`, `filecopy`, `filerename`,
+`filedeleteall` and `notemodify` commands
+may be included to update the contents of the branch prior to
+creating the commit.  These commands may be supplied in any order.
+However it is recommended that a `filedeleteall` command precede
+all `filemodify`, `filecopy`, `filerename` and `notemodify` commands in
+the same commit, as `filedeleteall` wipes the branch clean (see below).
+
+The `LF` after the command is optional (it used to be required).  Note
+that for reasons of backward compatibility, if the commit ends with a
+`data` command (i.e. it has no `from`, `merge`, `filemodify`,
+`filedelete`, `filecopy`, `filerename`, `filedeleteall` or
+`notemodify` commands) then two `LF` commands may appear at the end of
+the command instead of just one.
+
+`author`
+^^^^^^^^
+An `author` command may optionally appear, if the author information
+might differ from the committer information.  If `author` is omitted
+then fast-import will automatically use the committer's information for
+the author portion of the commit.  See below for a description of
+the fields in `author`, as they are identical to `committer`.
+
+`committer`
+^^^^^^^^^^^
+The `committer` command indicates who made this commit, and when
+they made it.
+
+Here `<name>` is the person's display name (for example
+``Com M Itter'') and `<email>` is the person's email address
+(``\cm@example.com'').  `LT` and `GT` are the literal less-than (\x3c)
+and greater-than (\x3e) symbols.  These are required to delimit
+the email address from the other fields in the line.  Note that
+`<name>` and `<email>` are free-form and may contain any sequence
+of bytes, except `LT`, `GT` and `LF`.  `<name>` is typically UTF-8 encoded.
+
+The time of the change is specified by `<when>` using the date format
+that was selected by the --date-format=<fmt> command-line option.
+See ``Date Formats'' above for the set of supported formats, and
+their syntax.
+
+`encoding`
+^^^^^^^^^^
+The optional `encoding` command indicates the encoding of the commit
+message.  Most commits are UTF-8 and the encoding is omitted, but this
+allows importing commit messages into git without first reencoding them.
+
+`from`
+^^^^^^
+The `from` command is used to specify the commit to initialize
+this branch from.  This revision will be the first ancestor of the
+new commit.  The state of the tree built at this commit will begin
+with the state at the `from` commit, and be altered by the content
+modifications in this commit.
+
+Omitting the `from` command in the first commit of a new branch
+will cause fast-import to create that commit with no ancestor. This
+tends to be desired only for the initial commit of a project.
+If the frontend creates all files from scratch when making a new
+branch, a `merge` command may be used instead of `from` to start
+the commit with an empty tree.
+Omitting the `from` command on existing branches is usually desired,
+as the current commit on that branch is automatically assumed to
+be the first ancestor of the new commit.
+
+As `LF` is not valid in a Git refname or SHA-1 expression, no
+quoting or escaping syntax is supported within `<commit-ish>`.
+
+Here `<commit-ish>` is any of the following:
+
+* The name of an existing branch already in fast-import's internal branch
+  table.  If fast-import doesn't know the name, it's treated as a SHA-1
+  expression.
+
+* A mark reference, `:<idnum>`, where `<idnum>` is the mark number.
++
+The reason fast-import uses `:` to denote a mark reference is this character
+is not legal in a Git branch name.  The leading `:` makes it easy
+to distinguish between the mark 42 (`:42`) and the branch 42 (`42`
+or `refs/heads/42`), or an abbreviated SHA-1 which happened to
+consist only of base-10 digits.
++
+Marks must be declared (via `mark`) before they can be used.
+
+* A complete 40 byte or abbreviated commit SHA-1 in hex.
+
+* Any valid Git SHA-1 expression that resolves to a commit.  See
+  ``SPECIFYING REVISIONS'' in linkgit:gitrevisions[7] for details.
+
+* The special null SHA-1 (40 zeros) specifies that the branch is to be
+  removed.
+
+The special case of restarting an incremental import from the
+current branch value should be written as:
+----
+	from refs/heads/branch^0
+----
+The `^0` suffix is necessary as fast-import does not permit a branch to
+start from itself, and the branch is created in memory before the
+`from` command is even read from the input.  Adding `^0` will force
+fast-import to resolve the commit through Git's revision parsing library,
+rather than its internal branch table, thereby loading in the
+existing value of the branch.
+
+`merge`
+^^^^^^^
+Includes one additional ancestor commit.  The additional ancestry
+link does not change the way the tree state is built at this commit.
+If the `from` command is
+omitted when creating a new branch, the first `merge` commit will be
+the first ancestor of the current commit, and the branch will start
+out with no files.  An unlimited number of `merge` commands per
+commit are permitted by fast-import, thereby establishing an n-way merge.
+
+Here `<commit-ish>` is any of the commit specification expressions
+also accepted by `from` (see above).
+
+`filemodify`
+^^^^^^^^^^^^
+Included in a `commit` command to add a new file or change the
+content of an existing file.  This command has two different means
+of specifying the content of the file.
+
+External data format::
+	The data content for the file was already supplied by a prior
+	`blob` command.  The frontend just needs to connect it.
++
+....
+	'M' SP <mode> SP <dataref> SP <path> LF
+....
++
+Here usually `<dataref>` must be either a mark reference (`:<idnum>`)
+set by a prior `blob` command, or a full 40-byte SHA-1 of an
+existing Git blob object.  If `<mode>` is `040000`` then
+`<dataref>` must be the full 40-byte SHA-1 of an existing
+Git tree object or a mark reference set with `--import-marks`.
+
+Inline data format::
+	The data content for the file has not been supplied yet.
+	The frontend wants to supply it as part of this modify
+	command.
++
+....
+	'M' SP <mode> SP 'inline' SP <path> LF
+	data
+....
++
+See below for a detailed description of the `data` command.
+
+In both formats `<mode>` is the type of file entry, specified
+in octal.  Git only supports the following modes:
+
+* `100644` or `644`: A normal (not-executable) file.  The majority
+  of files in most projects use this mode.  If in doubt, this is
+  what you want.
+* `100755` or `755`: A normal, but executable, file.
+* `120000`: A symlink, the content of the file will be the link target.
+* `160000`: A gitlink, SHA-1 of the object refers to a commit in
+  another repository. Git links can only be specified by SHA or through
+  a commit mark. They are used to implement submodules.
+* `040000`: A subdirectory.  Subdirectories can only be specified by
+  SHA or through a tree mark set with `--import-marks`.
+
+In both formats `<path>` is the complete path of the file to be added
+(if not already existing) or modified (if already existing).
+
+A `<path>` string must use UNIX-style directory separators (forward
+slash `/`), may contain any byte other than `LF`, and must not
+start with double quote (`"`).
+
+A path can use C-style string quoting; this is accepted in all cases
+and mandatory if the filename starts with double quote or contains
+`LF`. In C-style quoting, the complete name should be surrounded with
+double quotes, and any `LF`, backslash, or double quote characters
+must be escaped by preceding them with a backslash (e.g.,
+`"path/with\n, \\ and \" in it"`).
+
+The value of `<path>` must be in canonical form. That is it must not:
+
+* contain an empty directory component (e.g. `foo//bar` is invalid),
+* end with a directory separator (e.g. `foo/` is invalid),
+* start with a directory separator (e.g. `/foo` is invalid),
+* contain the special component `.` or `..` (e.g. `foo/./bar` and
+  `foo/../bar` are invalid).
+
+The root of the tree can be represented by an empty string as `<path>`.
+
+It is recommended that `<path>` always be encoded using UTF-8.
+
+`filedelete`
+^^^^^^^^^^^^
+Included in a `commit` command to remove a file or recursively
+delete an entire directory from the branch.  If the file or directory
+removal makes its parent directory empty, the parent directory will
+be automatically removed too.  This cascades up the tree until the
+first non-empty directory or the root is reached.
+
+....
+	'D' SP <path> LF
+....
+
+here `<path>` is the complete path of the file or subdirectory to
+be removed from the branch.
+See `filemodify` above for a detailed description of `<path>`.
+
+`filecopy`
+^^^^^^^^^^
+Recursively copies an existing file or subdirectory to a different
+location within the branch.  The existing file or directory must
+exist.  If the destination exists it will be completely replaced
+by the content copied from the source.
+
+....
+	'C' SP <path> SP <path> LF
+....
+
+here the first `<path>` is the source location and the second
+`<path>` is the destination.  See `filemodify` above for a detailed
+description of what `<path>` may look like.  To use a source path
+that contains SP the path must be quoted.
+
+A `filecopy` command takes effect immediately.  Once the source
+location has been copied to the destination any future commands
+applied to the source location will not impact the destination of
+the copy.
+
+`filerename`
+^^^^^^^^^^^^
+Renames an existing file or subdirectory to a different location
+within the branch.  The existing file or directory must exist. If
+the destination exists it will be replaced by the source directory.
+
+....
+	'R' SP <path> SP <path> LF
+....
+
+here the first `<path>` is the source location and the second
+`<path>` is the destination.  See `filemodify` above for a detailed
+description of what `<path>` may look like.  To use a source path
+that contains SP the path must be quoted.
+
+A `filerename` command takes effect immediately.  Once the source
+location has been renamed to the destination any future commands
+applied to the source location will create new files there and not
+impact the destination of the rename.
+
+Note that a `filerename` is the same as a `filecopy` followed by a
+`filedelete` of the source location.  There is a slight performance
+advantage to using `filerename`, but the advantage is so small
+that it is never worth trying to convert a delete/add pair in
+source material into a rename for fast-import.  This `filerename`
+command is provided just to simplify frontends that already have
+rename information and don't want bother with decomposing it into a
+`filecopy` followed by a `filedelete`.
+
+`filedeleteall`
+^^^^^^^^^^^^^^^
+Included in a `commit` command to remove all files (and also all
+directories) from the branch.  This command resets the internal
+branch structure to have no files in it, allowing the frontend
+to subsequently add all interesting files from scratch.
+
+....
+	'deleteall' LF
+....
+
+This command is extremely useful if the frontend does not know
+(or does not care to know) what files are currently on the branch,
+and therefore cannot generate the proper `filedelete` commands to
+update the content.
+
+Issuing a `filedeleteall` followed by the needed `filemodify`
+commands to set the correct content will produce the same results
+as sending only the needed `filemodify` and `filedelete` commands.
+The `filedeleteall` approach may however require fast-import to use slightly
+more memory per active branch (less than 1 MiB for even most large
+projects); so frontends that can easily obtain only the affected
+paths for a commit are encouraged to do so.
+
+`notemodify`
+^^^^^^^^^^^^
+Included in a `commit` `<notes_ref>` command to add a new note
+annotating a `<commit-ish>` or change this annotation contents.
+Internally it is similar to filemodify 100644 on `<commit-ish>`
+path (maybe split into subdirectories). It's not advised to
+use any other commands to write to the `<notes_ref>` tree except
+`filedeleteall` to delete all existing notes in this tree.
+This command has two different means of specifying the content
+of the note.
+
+External data format::
+	The data content for the note was already supplied by a prior
+	`blob` command.  The frontend just needs to connect it to the
+	commit that is to be annotated.
++
+....
+	'N' SP <dataref> SP <commit-ish> LF
+....
++
+Here `<dataref>` can be either a mark reference (`:<idnum>`)
+set by a prior `blob` command, or a full 40-byte SHA-1 of an
+existing Git blob object.
+
+Inline data format::
+	The data content for the note has not been supplied yet.
+	The frontend wants to supply it as part of this modify
+	command.
++
+....
+	'N' SP 'inline' SP <commit-ish> LF
+	data
+....
++
+See below for a detailed description of the `data` command.
+
+In both formats `<commit-ish>` is any of the commit specification
+expressions also accepted by `from` (see above).
+
+`mark`
+~~~~~~
+Arranges for fast-import to save a reference to the current object, allowing
+the frontend to recall this object at a future point in time, without
+knowing its SHA-1.  Here the current object is the object creation
+command the `mark` command appears within.  This can be `commit`,
+`tag`, and `blob`, but `commit` is the most common usage.
+
+....
+	'mark' SP ':' <idnum> LF
+....
+
+where `<idnum>` is the number assigned by the frontend to this mark.
+The value of `<idnum>` is expressed as an ASCII decimal integer.
+The value 0 is reserved and cannot be used as
+a mark.  Only values greater than or equal to 1 may be used as marks.
+
+New marks are created automatically.  Existing marks can be moved
+to another object simply by reusing the same `<idnum>` in another
+`mark` command.
+
+`original-oid`
+~~~~~~~~~~~~~~
+Provides the name of the object in the original source control system.
+fast-import will simply ignore this directive, but filter processes
+which operate on and modify the stream before feeding to fast-import
+may have uses for this information
+
+....
+	'original-oid' SP <object-identifier> LF
+....
+
+where `<object-identifer>` is any string not containing LF.
+
+`tag`
+~~~~~
+Creates an annotated tag referring to a specific commit.  To create
+lightweight (non-annotated) tags see the `reset` command below.
+
+....
+	'tag' SP <name> LF
+	'from' SP <commit-ish> LF
+	original-oid?
+	'tagger' (SP <name>)? SP LT <email> GT SP <when> LF
+	data
+....
+
+where `<name>` is the name of the tag to create.
+
+Tag names are automatically prefixed with `refs/tags/` when stored
+in Git, so importing the CVS branch symbol `RELENG-1_0-FINAL` would
+use just `RELENG-1_0-FINAL` for `<name>`, and fast-import will write the
+corresponding ref as `refs/tags/RELENG-1_0-FINAL`.
+
+The value of `<name>` must be a valid refname in Git and therefore
+may contain forward slashes.  As `LF` is not valid in a Git refname,
+no quoting or escaping syntax is supported here.
+
+The `from` command is the same as in the `commit` command; see
+above for details.
+
+The `tagger` command uses the same format as `committer` within
+`commit`; again see above for details.
+
+The `data` command following `tagger` must supply the annotated tag
+message (see below for `data` command syntax).  To import an empty
+tag message use a 0 length data.  Tag messages are free-form and are
+not interpreted by Git.  Currently they must be encoded in UTF-8,
+as fast-import does not permit other encodings to be specified.
+
+Signing annotated tags during import from within fast-import is not
+supported.  Trying to include your own PGP/GPG signature is not
+recommended, as the frontend does not (easily) have access to the
+complete set of bytes which normally goes into such a signature.
+If signing is required, create lightweight tags from within fast-import with
+`reset`, then create the annotated versions of those tags offline
+with the standard 'git tag' process.
+
+`reset`
+~~~~~~~
+Creates (or recreates) the named branch, optionally starting from
+a specific revision.  The reset command allows a frontend to issue
+a new `from` command for an existing branch, or to create a new
+branch from an existing commit without creating a new commit.
+
+....
+	'reset' SP <ref> LF
+	('from' SP <commit-ish> LF)?
+	LF?
+....
+
+For a detailed description of `<ref>` and `<commit-ish>` see above
+under `commit` and `from`.
+
+The `LF` after the command is optional (it used to be required).
+
+The `reset` command can also be used to create lightweight
+(non-annotated) tags.  For example:
+
+====
+	reset refs/tags/938
+	from :938
+====
+
+would create the lightweight tag `refs/tags/938` referring to
+whatever commit mark `:938` references.
+
+`blob`
+~~~~~~
+Requests writing one file revision to the packfile.  The revision
+is not connected to any commit; this connection must be formed in
+a subsequent `commit` command by referencing the blob through an
+assigned mark.
+
+....
+	'blob' LF
+	mark?
+	original-oid?
+	data
+....
+
+The mark command is optional here as some frontends have chosen
+to generate the Git SHA-1 for the blob on their own, and feed that
+directly to `commit`.  This is typically more work than it's worth
+however, as marks are inexpensive to store and easy to use.
+
+`data`
+~~~~~~
+Supplies raw data (for use as blob/file content, commit messages, or
+annotated tag messages) to fast-import.  Data can be supplied using an exact
+byte count or delimited with a terminating line.  Real frontends
+intended for production-quality conversions should always use the
+exact byte count format, as it is more robust and performs better.
+The delimited format is intended primarily for testing fast-import.
+
+Comment lines appearing within the `<raw>` part of `data` commands
+are always taken to be part of the body of the data and are therefore
+never ignored by fast-import.  This makes it safe to import any
+file/message content whose lines might start with `#`.
+
+Exact byte count format::
+	The frontend must specify the number of bytes of data.
++
+....
+	'data' SP <count> LF
+	<raw> LF?
+....
++
+where `<count>` is the exact number of bytes appearing within
+`<raw>`.  The value of `<count>` is expressed as an ASCII decimal
+integer.  The `LF` on either side of `<raw>` is not
+included in `<count>` and will not be included in the imported data.
++
+The `LF` after `<raw>` is optional (it used to be required) but
+recommended.  Always including it makes debugging a fast-import
+stream easier as the next command always starts in column 0
+of the next line, even if `<raw>` did not end with an `LF`.
+
+Delimited format::
+	A delimiter string is used to mark the end of the data.
+	fast-import will compute the length by searching for the delimiter.
+	This format is primarily useful for testing and is not
+	recommended for real data.
++
+....
+	'data' SP '<<' <delim> LF
+	<raw> LF
+	<delim> LF
+	LF?
+....
++
+where `<delim>` is the chosen delimiter string.  The string `<delim>`
+must not appear on a line by itself within `<raw>`, as otherwise
+fast-import will think the data ends earlier than it really does.  The `LF`
+immediately trailing `<raw>` is part of `<raw>`.  This is one of
+the limitations of the delimited format, it is impossible to supply
+a data chunk which does not have an LF as its last byte.
++
+The `LF` after `<delim> LF` is optional (it used to be required).
+
+`checkpoint`
+~~~~~~~~~~~~
+Forces fast-import to close the current packfile, start a new one, and to
+save out all current branch refs, tags and marks.
+
+....
+	'checkpoint' LF
+	LF?
+....
+
+Note that fast-import automatically switches packfiles when the current
+packfile reaches --max-pack-size, or 4 GiB, whichever limit is
+smaller.  During an automatic packfile switch fast-import does not update
+the branch refs, tags or marks.
+
+As a `checkpoint` can require a significant amount of CPU time and
+disk IO (to compute the overall pack SHA-1 checksum, generate the
+corresponding index file, and update the refs) it can easily take
+several minutes for a single `checkpoint` command to complete.
+
+Frontends may choose to issue checkpoints during extremely large
+and long running imports, or when they need to allow another Git
+process access to a branch.  However given that a 30 GiB Subversion
+repository can be loaded into Git through fast-import in about 3 hours,
+explicit checkpointing may not be necessary.
+
+The `LF` after the command is optional (it used to be required).
+
+`progress`
+~~~~~~~~~~
+Causes fast-import to print the entire `progress` line unmodified to
+its standard output channel (file descriptor 1) when the command is
+processed from the input stream.  The command otherwise has no impact
+on the current import, or on any of fast-import's internal state.
+
+....
+	'progress' SP <any> LF
+	LF?
+....
+
+The `<any>` part of the command may contain any sequence of bytes
+that does not contain `LF`.  The `LF` after the command is optional.
+Callers may wish to process the output through a tool such as sed to
+remove the leading part of the line, for example:
+
+====
+	frontend | git fast-import | sed 's/^progress //'
+====
+
+Placing a `progress` command immediately after a `checkpoint` will
+inform the reader when the `checkpoint` has been completed and it
+can safely access the refs that fast-import updated.
+
+`get-mark`
+~~~~~~~~~~
+Causes fast-import to print the SHA-1 corresponding to a mark to
+stdout or to the file descriptor previously arranged with the
+`--cat-blob-fd` argument. The command otherwise has no impact on the
+current import; its purpose is to retrieve SHA-1s that later commits
+might want to refer to in their commit messages.
+
+....
+	'get-mark' SP ':' <idnum> LF
+....
+
+See ``Responses To Commands'' below for details about how to read
+this output safely.
+
+`cat-blob`
+~~~~~~~~~~
+Causes fast-import to print a blob to a file descriptor previously
+arranged with the `--cat-blob-fd` argument.  The command otherwise
+has no impact on the current import; its main purpose is to
+retrieve blobs that may be in fast-import's memory but not
+accessible from the target repository.
+
+....
+	'cat-blob' SP <dataref> LF
+....
+
+The `<dataref>` can be either a mark reference (`:<idnum>`)
+set previously or a full 40-byte SHA-1 of a Git blob, preexisting or
+ready to be written.
+
+Output uses the same format as `git cat-file --batch`:
+
+====
+	<sha1> SP 'blob' SP <size> LF
+	<contents> LF
+====
+
+This command can be used where a `filemodify` directive can appear,
+allowing it to be used in the middle of a commit.  For a `filemodify`
+using an inline directive, it can also appear right before the `data`
+directive.
+
+See ``Responses To Commands'' below for details about how to read
+this output safely.
+
+`ls`
+~~~~
+Prints information about the object at a path to a file descriptor
+previously arranged with the `--cat-blob-fd` argument.  This allows
+printing a blob from the active commit (with `cat-blob`) or copying a
+blob or tree from a previous commit for use in the current one (with
+`filemodify`).
+
+The `ls` command can also be used where a `filemodify` directive can
+appear, allowing it to be used in the middle of a commit.
+
+Reading from the active commit::
+	This form can only be used in the middle of a `commit`.
+	The path names a directory entry within fast-import's
+	active commit.  The path must be quoted in this case.
++
+....
+	'ls' SP <path> LF
+....
+
+Reading from a named tree::
+	The `<dataref>` can be a mark reference (`:<idnum>`) or the
+	full 40-byte SHA-1 of a Git tag, commit, or tree object,
+	preexisting or waiting to be written.
+	The path is relative to the top level of the tree
+	named by `<dataref>`.
++
+....
+	'ls' SP <dataref> SP <path> LF
+....
+
+See `filemodify` above for a detailed description of `<path>`.
+
+Output uses the same format as `git ls-tree <tree> -- <path>`:
+
+====
+	<mode> SP ('blob' | 'tree' | 'commit') SP <dataref> HT <path> LF
+====
+
+The <dataref> represents the blob, tree, or commit object at <path>
+and can be used in later 'get-mark', 'cat-blob', 'filemodify', or
+'ls' commands.
+
+If there is no file or subtree at that path, 'git fast-import' will
+instead report
+
+====
+	missing SP <path> LF
+====
+
+See ``Responses To Commands'' below for details about how to read
+this output safely.
+
+`feature`
+~~~~~~~~~
+Require that fast-import supports the specified feature, or abort if
+it does not.
+
+....
+	'feature' SP <feature> ('=' <argument>)? LF
+....
+
+The <feature> part of the command may be any one of the following:
+
+date-format::
+export-marks::
+relative-marks::
+no-relative-marks::
+force::
+	Act as though the corresponding command-line option with
+	a leading `--` was passed on the command line
+	(see OPTIONS, above).
+
+import-marks::
+import-marks-if-exists::
+	Like --import-marks except in two respects: first, only one
+	"feature import-marks" or "feature import-marks-if-exists"
+	command is allowed per stream; second, an --import-marks=
+	or --import-marks-if-exists command-line option overrides
+	any of these "feature" commands in the stream; third,
+	"feature import-marks-if-exists" like a corresponding
+	command-line option silently skips a nonexistent file.
+
+get-mark::
+cat-blob::
+ls::
+	Require that the backend support the 'get-mark', 'cat-blob',
+	or 'ls' command respectively.
+	Versions of fast-import not supporting the specified command
+	will exit with a message indicating so.
+	This lets the import error out early with a clear message,
+	rather than wasting time on the early part of an import
+	before the unsupported command is detected.
+
+notes::
+	Require that the backend support the 'notemodify' (N)
+	subcommand to the 'commit' command.
+	Versions of fast-import not supporting notes will exit
+	with a message indicating so.
+
+done::
+	Error out if the stream ends without a 'done' command.
+	Without this feature, errors causing the frontend to end
+	abruptly at a convenient point in the stream can go
+	undetected.  This may occur, for example, if an import
+	front end dies in mid-operation without emitting SIGTERM
+	or SIGKILL at its subordinate git fast-import instance.
+
+`option`
+~~~~~~~~
+Processes the specified option so that git fast-import behaves in a
+way that suits the frontend's needs.
+Note that options specified by the frontend are overridden by any
+options the user may specify to git fast-import itself.
+
+....
+    'option' SP <option> LF
+....
+
+The `<option>` part of the command may contain any of the options
+listed in the OPTIONS section that do not change import semantics,
+without the leading `--` and is treated in the same way.
+
+Option commands must be the first commands on the input (not counting
+feature commands), to give an option command after any non-option
+command is an error.
+
+The following command-line options change import semantics and may therefore
+not be passed as option:
+
+* date-format
+* import-marks
+* export-marks
+* cat-blob-fd
+* force
+
+`done`
+~~~~~~
+If the `done` feature is not in use, treated as if EOF was read.
+This can be used to tell fast-import to finish early.
+
+If the `--done` command-line option or `feature done` command is
+in use, the `done` command is mandatory and marks the end of the
+stream.
+
+RESPONSES TO COMMANDS
+---------------------
+New objects written by fast-import are not available immediately.
+Most fast-import commands have no visible effect until the next
+checkpoint (or completion).  The frontend can send commands to
+fill fast-import's input pipe without worrying about how quickly
+they will take effect, which improves performance by simplifying
+scheduling.
+
+For some frontends, though, it is useful to be able to read back
+data from the current repository as it is being updated (for
+example when the source material describes objects in terms of
+patches to be applied to previously imported objects).  This can
+be accomplished by connecting the frontend and fast-import via
+bidirectional pipes:
+
+====
+	mkfifo fast-import-output
+	frontend <fast-import-output |
+	git fast-import >fast-import-output
+====
+
+A frontend set up this way can use `progress`, `get-mark`, `ls`, and
+`cat-blob` commands to read information from the import in progress.
+
+To avoid deadlock, such frontends must completely consume any
+pending output from `progress`, `ls`, `get-mark`, and `cat-blob` before
+performing writes to fast-import that might block.
+
+CRASH REPORTS
+-------------
+If fast-import is supplied invalid input it will terminate with a
+non-zero exit status and create a crash report in the top level of
+the Git repository it was importing into.  Crash reports contain
+a snapshot of the internal fast-import state as well as the most
+recent commands that lead up to the crash.
+
+All recent commands (including stream comments, file changes and
+progress commands) are shown in the command history within the crash
+report, but raw file data and commit messages are excluded from the
+crash report.  This exclusion saves space within the report file
+and reduces the amount of buffering that fast-import must perform
+during execution.
+
+After writing a crash report fast-import will close the current
+packfile and export the marks table.  This allows the frontend
+developer to inspect the repository state and resume the import from
+the point where it crashed.  The modified branches and tags are not
+updated during a crash, as the import did not complete successfully.
+Branch and tag information can be found in the crash report and
+must be applied manually if the update is needed.
+
+An example crash:
+
+====
+	$ cat >in <<END_OF_INPUT
+	# my very first test commit
+	commit refs/heads/master
+	committer Shawn O. Pearce <spearce> 19283 -0400
+	# who is that guy anyway?
+	data <<EOF
+	this is my commit
+	EOF
+	M 644 inline .gitignore
+	data <<EOF
+	.gitignore
+	EOF
+	M 777 inline bob
+	END_OF_INPUT
+
+	$ git fast-import <in
+	fatal: Corrupt mode: M 777 inline bob
+	fast-import: dumping crash report to .git/fast_import_crash_8434
+
+	$ cat .git/fast_import_crash_8434
+	fast-import crash report:
+	    fast-import process: 8434
+	    parent process     : 1391
+	    at Sat Sep 1 00:58:12 2007
+
+	fatal: Corrupt mode: M 777 inline bob
+
+	Most Recent Commands Before Crash
+	---------------------------------
+	  # my very first test commit
+	  commit refs/heads/master
+	  committer Shawn O. Pearce <spearce> 19283 -0400
+	  # who is that guy anyway?
+	  data <<EOF
+	  M 644 inline .gitignore
+	  data <<EOF
+	* M 777 inline bob
+
+	Active Branch LRU
+	-----------------
+	    active_branches = 1 cur, 5 max
+
+	  pos  clock name
+	  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+	   1)      0 refs/heads/master
+
+	Inactive Branches
+	-----------------
+	refs/heads/master:
+	  status      : active loaded dirty
+	  tip commit  : 0000000000000000000000000000000000000000
+	  old tree    : 0000000000000000000000000000000000000000
+	  cur tree    : 0000000000000000000000000000000000000000
+	  commit clock: 0
+	  last pack   :
+
+
+	-------------------
+	END OF CRASH REPORT
+====
+
+TIPS AND TRICKS
+---------------
+The following tips and tricks have been collected from various
+users of fast-import, and are offered here as suggestions.
+
+Use One Mark Per Commit
+~~~~~~~~~~~~~~~~~~~~~~~
+When doing a repository conversion, use a unique mark per commit
+(`mark :<n>`) and supply the --export-marks option on the command
+line.  fast-import will dump a file which lists every mark and the Git
+object SHA-1 that corresponds to it.  If the frontend can tie
+the marks back to the source repository, it is easy to verify the
+accuracy and completeness of the import by comparing each Git
+commit to the corresponding source revision.
+
+Coming from a system such as Perforce or Subversion this should be
+quite simple, as the fast-import mark can also be the Perforce changeset
+number or the Subversion revision number.
+
+Freely Skip Around Branches
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Don't bother trying to optimize the frontend to stick to one branch
+at a time during an import.  Although doing so might be slightly
+faster for fast-import, it tends to increase the complexity of the frontend
+code considerably.
+
+The branch LRU builtin to fast-import tends to behave very well, and the
+cost of activating an inactive branch is so low that bouncing around
+between branches has virtually no impact on import performance.
+
+Handling Renames
+~~~~~~~~~~~~~~~~
+When importing a renamed file or directory, simply delete the old
+name(s) and modify the new name(s) during the corresponding commit.
+Git performs rename detection after-the-fact, rather than explicitly
+during a commit.
+
+Use Tag Fixup Branches
+~~~~~~~~~~~~~~~~~~~~~~
+Some other SCM systems let the user create a tag from multiple
+files which are not from the same commit/changeset.  Or to create
+tags which are a subset of the files available in the repository.
+
+Importing these tags as-is in Git is impossible without making at
+least one commit which ``fixes up'' the files to match the content
+of the tag.  Use fast-import's `reset` command to reset a dummy branch
+outside of your normal branch space to the base commit for the tag,
+then commit one or more file fixup commits, and finally tag the
+dummy branch.
+
+For example since all normal branches are stored under `refs/heads/`
+name the tag fixup branch `TAG_FIXUP`.  This way it is impossible for
+the fixup branch used by the importer to have namespace conflicts
+with real branches imported from the source (the name `TAG_FIXUP`
+is not `refs/heads/TAG_FIXUP`).
+
+When committing fixups, consider using `merge` to connect the
+commit(s) which are supplying file revisions to the fixup branch.
+Doing so will allow tools such as 'git blame' to track
+through the real commit history and properly annotate the source
+files.
+
+After fast-import terminates the frontend will need to do `rm .git/TAG_FIXUP`
+to remove the dummy branch.
+
+Import Now, Repack Later
+~~~~~~~~~~~~~~~~~~~~~~~~
+As soon as fast-import completes the Git repository is completely valid
+and ready for use.  Typically this takes only a very short time,
+even for considerably large projects (100,000+ commits).
+
+However repacking the repository is necessary to improve data
+locality and access performance.  It can also take hours on extremely
+large projects (especially if -f and a large --window parameter is
+used).  Since repacking is safe to run alongside readers and writers,
+run the repack in the background and let it finish when it finishes.
+There is no reason to wait to explore your new Git project!
+
+If you choose to wait for the repack, don't try to run benchmarks
+or performance tests until repacking is completed.  fast-import outputs
+suboptimal packfiles that are simply never seen in real use
+situations.
+
+Repacking Historical Data
+~~~~~~~~~~~~~~~~~~~~~~~~~
+If you are repacking very old imported data (e.g. older than the
+last year), consider expending some extra CPU time and supplying
+--window=50 (or higher) when you run 'git repack'.
+This will take longer, but will also produce a smaller packfile.
+You only need to expend the effort once, and everyone using your
+project will benefit from the smaller repository.
+
+Include Some Progress Messages
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Every once in a while have your frontend emit a `progress` message
+to fast-import.  The contents of the messages are entirely free-form,
+so one suggestion would be to output the current month and year
+each time the current commit date moves into the next month.
+Your users will feel better knowing how much of the data stream
+has been processed.
+
+
+PACKFILE OPTIMIZATION
+---------------------
+When packing a blob fast-import always attempts to deltify against the last
+blob written.  Unless specifically arranged for by the frontend,
+this will probably not be a prior version of the same file, so the
+generated delta will not be the smallest possible.  The resulting
+packfile will be compressed, but will not be optimal.
+
+Frontends which have efficient access to all revisions of a
+single file (for example reading an RCS/CVS ,v file) can choose
+to supply all revisions of that file as a sequence of consecutive
+`blob` commands.  This allows fast-import to deltify the different file
+revisions against each other, saving space in the final packfile.
+Marks can be used to later identify individual file revisions during
+a sequence of `commit` commands.
+
+The packfile(s) created by fast-import do not encourage good disk access
+patterns.  This is caused by fast-import writing the data in the order
+it is received on standard input, while Git typically organizes
+data within packfiles to make the most recent (current tip) data
+appear before historical data.  Git also clusters commits together,
+speeding up revision traversal through better cache locality.
+
+For this reason it is strongly recommended that users repack the
+repository with `git repack -a -d` after fast-import completes, allowing
+Git to reorganize the packfiles for faster data access.  If blob
+deltas are suboptimal (see above) then also adding the `-f` option
+to force recomputation of all deltas can significantly reduce the
+final packfile size (30-50% smaller can be quite typical).
+
+Instead of running `git repack` you can also run `git gc
+--aggressive`, which will also optimize other things after an import
+(e.g. pack loose refs). As noted in the "AGGRESSIVE" section in
+linkgit:git-gc[1] the `--aggressive` option will find new deltas with
+the `-f` option to linkgit:git-repack[1]. For the reasons elaborated
+on above using `--aggressive` after a fast-import is one of the few
+cases where it's known to be worthwhile.
+
+MEMORY UTILIZATION
+------------------
+There are a number of factors which affect how much memory fast-import
+requires to perform an import.  Like critical sections of core
+Git, fast-import uses its own memory allocators to amortize any overheads
+associated with malloc.  In practice fast-import tends to amortize any
+malloc overheads to 0, due to its use of large block allocations.
+
+per object
+~~~~~~~~~~
+fast-import maintains an in-memory structure for every object written in
+this execution.  On a 32 bit system the structure is 32 bytes,
+on a 64 bit system the structure is 40 bytes (due to the larger
+pointer sizes).  Objects in the table are not deallocated until
+fast-import terminates.  Importing 2 million objects on a 32 bit system
+will require approximately 64 MiB of memory.
+
+The object table is actually a hashtable keyed on the object name
+(the unique SHA-1).  This storage configuration allows fast-import to reuse
+an existing or already written object and avoid writing duplicates
+to the output packfile.  Duplicate blobs are surprisingly common
+in an import, typically due to branch merges in the source.
+
+per mark
+~~~~~~~~
+Marks are stored in a sparse array, using 1 pointer (4 bytes or 8
+bytes, depending on pointer size) per mark.  Although the array
+is sparse, frontends are still strongly encouraged to use marks
+between 1 and n, where n is the total number of marks required for
+this import.
+
+per branch
+~~~~~~~~~~
+Branches are classified as active and inactive.  The memory usage
+of the two classes is significantly different.
+
+Inactive branches are stored in a structure which uses 96 or 120
+bytes (32 bit or 64 bit systems, respectively), plus the length of
+the branch name (typically under 200 bytes), per branch.  fast-import will
+easily handle as many as 10,000 inactive branches in under 2 MiB
+of memory.
+
+Active branches have the same overhead as inactive branches, but
+also contain copies of every tree that has been recently modified on
+that branch.  If subtree `include` has not been modified since the
+branch became active, its contents will not be loaded into memory,
+but if subtree `src` has been modified by a commit since the branch
+became active, then its contents will be loaded in memory.
+
+As active branches store metadata about the files contained on that
+branch, their in-memory storage size can grow to a considerable size
+(see below).
+
+fast-import automatically moves active branches to inactive status based on
+a simple least-recently-used algorithm.  The LRU chain is updated on
+each `commit` command.  The maximum number of active branches can be
+increased or decreased on the command line with --active-branches=.
+
+per active tree
+~~~~~~~~~~~~~~~
+Trees (aka directories) use just 12 bytes of memory on top of the
+memory required for their entries (see ``per active file'' below).
+The cost of a tree is virtually 0, as its overhead amortizes out
+over the individual file entries.
+
+per active file entry
+~~~~~~~~~~~~~~~~~~~~~
+Files (and pointers to subtrees) within active trees require 52 or 64
+bytes (32/64 bit platforms) per entry.  To conserve space, file and
+tree names are pooled in a common string table, allowing the filename
+``Makefile'' to use just 16 bytes (after including the string header
+overhead) no matter how many times it occurs within the project.
+
+The active branch LRU, when coupled with the filename string pool
+and lazy loading of subtrees, allows fast-import to efficiently import
+projects with 2,000+ branches and 45,114+ files in a very limited
+memory footprint (less than 2.7 MiB per active branch).
+
+SIGNALS
+-------
+Sending *SIGUSR1* to the 'git fast-import' process ends the current
+packfile early, simulating a `checkpoint` command.  The impatient
+operator can use this facility to peek at the objects and refs from an
+import in progress, at the cost of some added running time and worse
+compression.
+
+SEE ALSO
+--------
+linkgit:git-fast-export[1]
+
+GIT
+---
+Part of the linkgit:git[1] suite
-- 
cgit 1.4.1