merge(third_party/git): Merge squashed git subtree at v2.23.0 r/373

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+git-filter-branch(1)
+====================
+
+NAME
+----
+git-filter-branch - Rewrite branches
+
+SYNOPSIS
+--------
+[verse]
+'git filter-branch' [--setup <command>] [--subdirectory-filter <directory>]
+	[--env-filter <command>] [--tree-filter <command>]
+	[--index-filter <command>] [--parent-filter <command>]
+	[--msg-filter <command>] [--commit-filter <command>]
+	[--tag-name-filter <command>] [--prune-empty]
+	[--original <namespace>] [-d <directory>] [-f | --force]
+	[--state-branch <branch>] [--] [<rev-list options>...]
+
+DESCRIPTION
+-----------
+Lets you rewrite Git revision history by rewriting the branches mentioned
+in the <rev-list options>, applying custom filters on each revision.
+Those filters can modify each tree (e.g. removing a file or running
+a perl rewrite on all files) or information about each commit.
+Otherwise, all information (including original commit times or merge
+information) will be preserved.
+
+The command will only rewrite the _positive_ refs mentioned in the
+command line (e.g. if you pass 'a..b', only 'b' will be rewritten).
+If you specify no filters, the commits will be recommitted without any
+changes, which would normally have no effect.  Nevertheless, this may be
+useful in the future for compensating for some Git bugs or such,
+therefore such a usage is permitted.
+
+*NOTE*: This command honors `.git/info/grafts` file and refs in
+the `refs/replace/` namespace.
+If you have any grafts or replacement refs defined, running this command
+will make them permanent.
+
+*WARNING*! The rewritten history will have different object names for all
+the objects and will not converge with the original branch.  You will not
+be able to easily push and distribute the rewritten branch on top of the
+original branch.  Please do not use this command if you do not know the
+full implications, and avoid using it anyway, if a simple single commit
+would suffice to fix your problem.  (See the "RECOVERING FROM UPSTREAM
+REBASE" section in linkgit:git-rebase[1] for further information about
+rewriting published history.)
+
+Always verify that the rewritten version is correct: The original refs,
+if different from the rewritten ones, will be stored in the namespace
+'refs/original/'.
+
+Note that since this operation is very I/O expensive, it might
+be a good idea to redirect the temporary directory off-disk with the
+`-d` option, e.g. on tmpfs.  Reportedly the speedup is very noticeable.
+
+
+Filters
+~~~~~~~
+
+The filters are applied in the order as listed below.  The <command>
+argument is always evaluated in the shell context using the 'eval' command
+(with the notable exception of the commit filter, for technical reasons).
+Prior to that, the `$GIT_COMMIT` environment variable will be set to contain
+the id of the commit being rewritten.  Also, GIT_AUTHOR_NAME,
+GIT_AUTHOR_EMAIL, GIT_AUTHOR_DATE, GIT_COMMITTER_NAME, GIT_COMMITTER_EMAIL,
+and GIT_COMMITTER_DATE are taken from the current commit and exported to
+the environment, in order to affect the author and committer identities of
+the replacement commit created by linkgit:git-commit-tree[1] after the
+filters have run.
+
+If any evaluation of <command> returns a non-zero exit status, the whole
+operation will be aborted.
+
+A 'map' function is available that takes an "original sha1 id" argument
+and outputs a "rewritten sha1 id" if the commit has been already
+rewritten, and "original sha1 id" otherwise; the 'map' function can
+return several ids on separate lines if your commit filter emitted
+multiple commits.
+
+
+OPTIONS
+-------
+
+--setup <command>::
+	This is not a real filter executed for each commit but a one
+	time setup just before the loop. Therefore no commit-specific
+	variables are defined yet.  Functions or variables defined here
+	can be used or modified in the following filter steps except
+	the commit filter, for technical reasons.
+
+--subdirectory-filter <directory>::
+	Only look at the history which touches the given subdirectory.
+	The result will contain that directory (and only that) as its
+	project root. Implies <<Remap_to_ancestor>>.
+
+--env-filter <command>::
+	This filter may be used if you only need to modify the environment
+	in which the commit will be performed.  Specifically, you might
+	want to rewrite the author/committer name/email/time environment
+	variables (see linkgit:git-commit-tree[1] for details).
+
+--tree-filter <command>::
+	This is the filter for rewriting the tree and its contents.
+	The argument is evaluated in shell with the working
+	directory set to the root of the checked out tree.  The new tree
+	is then used as-is (new files are auto-added, disappeared files
+	are auto-removed - neither .gitignore files nor any other ignore
+	rules *HAVE ANY EFFECT*!).
+
+--index-filter <command>::
+	This is the filter for rewriting the index.  It is similar to the
+	tree filter but does not check out the tree, which makes it much
+	faster.  Frequently used with `git rm --cached
+	--ignore-unmatch ...`, see EXAMPLES below.  For hairy
+	cases, see linkgit:git-update-index[1].
+
+--parent-filter <command>::
+	This is the filter for rewriting the commit's parent list.
+	It will receive the parent string on stdin and shall output
+	the new parent string on stdout.  The parent string is in
+	the format described in linkgit:git-commit-tree[1]: empty for
+	the initial commit, "-p parent" for a normal commit and
+	"-p parent1 -p parent2 -p parent3 ..." for a merge commit.
+
+--msg-filter <command>::
+	This is the filter for rewriting the commit messages.
+	The argument is evaluated in the shell with the original
+	commit message on standard input; its standard output is
+	used as the new commit message.
+
+--commit-filter <command>::
+	This is the filter for performing the commit.
+	If this filter is specified, it will be called instead of the
+	'git commit-tree' command, with arguments of the form
+	"<TREE_ID> [(-p <PARENT_COMMIT_ID>)...]" and the log message on
+	stdin.  The commit id is expected on stdout.
++
+As a special extension, the commit filter may emit multiple
+commit ids; in that case, the rewritten children of the original commit will
+have all of them as parents.
++
+You can use the 'map' convenience function in this filter, and other
+convenience functions, too.  For example, calling 'skip_commit "$@"'
+will leave out the current commit (but not its changes! If you want
+that, use 'git rebase' instead).
++
+You can also use the `git_commit_non_empty_tree "$@"` instead of
+`git commit-tree "$@"` if you don't wish to keep commits with a single parent
+and that makes no change to the tree.
+
+--tag-name-filter <command>::
+	This is the filter for rewriting tag names. When passed,
+	it will be called for every tag ref that points to a rewritten
+	object (or to a tag object which points to a rewritten object).
+	The original tag name is passed via standard input, and the new
+	tag name is expected on standard output.
++
+The original tags are not deleted, but can be overwritten;
+use "--tag-name-filter cat" to simply update the tags.  In this
+case, be very careful and make sure you have the old tags
+backed up in case the conversion has run afoul.
++
+Nearly proper rewriting of tag objects is supported. If the tag has
+a message attached, a new tag object will be created with the same message,
+author, and timestamp. If the tag has a signature attached, the
+signature will be stripped. It is by definition impossible to preserve
+signatures. The reason this is "nearly" proper, is because ideally if
+the tag did not change (points to the same object, has the same name, etc.)
+it should retain any signature. That is not the case, signatures will always
+be removed, buyer beware. There is also no support for changing the
+author or timestamp (or the tag message for that matter). Tags which point
+to other tags will be rewritten to point to the underlying commit.
+
+--prune-empty::
+	Some filters will generate empty commits that leave the tree untouched.
+	This option instructs git-filter-branch to remove such commits if they
+	have exactly one or zero non-pruned parents; merge commits will
+	therefore remain intact.  This option cannot be used together with
+	`--commit-filter`, though the same effect can be achieved by using the
+	provided `git_commit_non_empty_tree` function in a commit filter.
+
+--original <namespace>::
+	Use this option to set the namespace where the original commits
+	will be stored. The default value is 'refs/original'.
+
+-d <directory>::
+	Use this option to set the path to the temporary directory used for
+	rewriting.  When applying a tree filter, the command needs to
+	temporarily check out the tree to some directory, which may consume
+	considerable space in case of large projects.  By default it
+	does this in the `.git-rewrite/` directory but you can override
+	that choice by this parameter.
+
+-f::
+--force::
+	'git filter-branch' refuses to start with an existing temporary
+	directory or when there are already refs starting with
+	'refs/original/', unless forced.
+
+--state-branch <branch>::
+	This option will cause the mapping from old to new objects to
+	be loaded from named branch upon startup and saved as a new
+	commit to that branch upon exit, enabling incremental of large
+	trees. If '<branch>' does not exist it will be created.
+
+<rev-list options>...::
+	Arguments for 'git rev-list'.  All positive refs included by
+	these options are rewritten.  You may also specify options
+	such as `--all`, but you must use `--` to separate them from
+	the 'git filter-branch' options. Implies <<Remap_to_ancestor>>.
+
+
+[[Remap_to_ancestor]]
+Remap to ancestor
+~~~~~~~~~~~~~~~~~
+
+By using linkgit:git-rev-list[1] arguments, e.g., path limiters, you can limit the
+set of revisions which get rewritten. However, positive refs on the command
+line are distinguished: we don't let them be excluded by such limiters. For
+this purpose, they are instead rewritten to point at the nearest ancestor that
+was not excluded.
+
+
+EXIT STATUS
+-----------
+
+On success, the exit status is `0`.  If the filter can't find any commits to
+rewrite, the exit status is `2`.  On any other error, the exit status may be
+any other non-zero value.
+
+
+EXAMPLES
+--------
+
+Suppose you want to remove a file (containing confidential information
+or copyright violation) from all commits:
+
+-------------------------------------------------------
+git filter-branch --tree-filter 'rm filename' HEAD
+-------------------------------------------------------
+
+However, if the file is absent from the tree of some commit,
+a simple `rm filename` will fail for that tree and commit.
+Thus you may instead want to use `rm -f filename` as the script.
+
+Using `--index-filter` with 'git rm' yields a significantly faster
+version.  Like with using `rm filename`, `git rm --cached filename`
+will fail if the file is absent from the tree of a commit.  If you
+want to "completely forget" a file, it does not matter when it entered
+history, so we also add `--ignore-unmatch`:
+
+--------------------------------------------------------------------------
+git filter-branch --index-filter 'git rm --cached --ignore-unmatch filename' HEAD
+--------------------------------------------------------------------------
+
+Now, you will get the rewritten history saved in HEAD.
+
+To rewrite the repository to look as if `foodir/` had been its project
+root, and discard all other history:
+
+-------------------------------------------------------
+git filter-branch --subdirectory-filter foodir -- --all
+-------------------------------------------------------
+
+Thus you can, e.g., turn a library subdirectory into a repository of
+its own.  Note the `--` that separates 'filter-branch' options from
+revision options, and the `--all` to rewrite all branches and tags.
+
+To set a commit (which typically is at the tip of another
+history) to be the parent of the current initial commit, in
+order to paste the other history behind the current history:
+
+-------------------------------------------------------------------
+git filter-branch --parent-filter 'sed "s/^\$/-p <graft-id>/"' HEAD
+-------------------------------------------------------------------
+
+(if the parent string is empty - which happens when we are dealing with
+the initial commit - add graftcommit as a parent).  Note that this assumes
+history with a single root (that is, no merge without common ancestors
+happened).  If this is not the case, use:
+
+--------------------------------------------------------------------------
+git filter-branch --parent-filter \
+	'test $GIT_COMMIT = <commit-id> && echo "-p <graft-id>" || cat' HEAD
+--------------------------------------------------------------------------
+
+or even simpler:
+
+-----------------------------------------------
+git replace --graft $commit-id $graft-id
+git filter-branch $graft-id..HEAD
+-----------------------------------------------
+
+To remove commits authored by "Darl McBribe" from the history:
+
+------------------------------------------------------------------------------
+git filter-branch --commit-filter '
+	if [ "$GIT_AUTHOR_NAME" = "Darl McBribe" ];
+	then
+		skip_commit "$@";
+	else
+		git commit-tree "$@";
+	fi' HEAD
+------------------------------------------------------------------------------
+
+The function 'skip_commit' is defined as follows:
+
+--------------------------
+skip_commit()
+{
+	shift;
+	while [ -n "$1" ];
+	do
+		shift;
+		map "$1";
+		shift;
+	done;
+}
+--------------------------
+
+The shift magic first throws away the tree id and then the -p
+parameters.  Note that this handles merges properly! In case Darl
+committed a merge between P1 and P2, it will be propagated properly
+and all children of the merge will become merge commits with P1,P2
+as their parents instead of the merge commit.
+
+*NOTE* the changes introduced by the commits, and which are not reverted
+by subsequent commits, will still be in the rewritten branch. If you want
+to throw out _changes_ together with the commits, you should use the
+interactive mode of 'git rebase'.
+
+You can rewrite the commit log messages using `--msg-filter`.  For
+example, 'git svn-id' strings in a repository created by 'git svn' can
+be removed this way:
+
+-------------------------------------------------------
+git filter-branch --msg-filter '
+	sed -e "/^git-svn-id:/d"
+'
+-------------------------------------------------------
+
+If you need to add 'Acked-by' lines to, say, the last 10 commits (none
+of which is a merge), use this command:
+
+--------------------------------------------------------
+git filter-branch --msg-filter '
+	cat &&
+	echo "Acked-by: Bugs Bunny <bunny@bugzilla.org>"
+' HEAD~10..HEAD
+--------------------------------------------------------
+
+The `--env-filter` option can be used to modify committer and/or author
+identity.  For example, if you found out that your commits have the wrong
+identity due to a misconfigured user.email, you can make a correction,
+before publishing the project, like this:
+
+--------------------------------------------------------
+git filter-branch --env-filter '
+	if test "$GIT_AUTHOR_EMAIL" = "root@localhost"
+	then
+		GIT_AUTHOR_EMAIL=john@example.com
+	fi
+	if test "$GIT_COMMITTER_EMAIL" = "root@localhost"
+	then
+		GIT_COMMITTER_EMAIL=john@example.com
+	fi
+' -- --all
+--------------------------------------------------------
+
+To restrict rewriting to only part of the history, specify a revision
+range in addition to the new branch name.  The new branch name will
+point to the top-most revision that a 'git rev-list' of this range
+will print.
+
+Consider this history:
+
+------------------
+     D--E--F--G--H
+    /     /
+A--B-----C
+------------------
+
+To rewrite only commits D,E,F,G,H, but leave A, B and C alone, use:
+
+--------------------------------
+git filter-branch ... C..H
+--------------------------------
+
+To rewrite commits E,F,G,H, use one of these:
+
+----------------------------------------
+git filter-branch ... C..H --not D
+git filter-branch ... D..H --not C
+----------------------------------------
+
+To move the whole tree into a subdirectory, or remove it from there:
+
+---------------------------------------------------------------
+git filter-branch --index-filter \
+	'git ls-files -s | sed "s-\t\"*-&newsubdir/-" |
+		GIT_INDEX_FILE=$GIT_INDEX_FILE.new \
+			git update-index --index-info &&
+	 mv "$GIT_INDEX_FILE.new" "$GIT_INDEX_FILE"' HEAD
+---------------------------------------------------------------
+
+
+
+CHECKLIST FOR SHRINKING A REPOSITORY
+------------------------------------
+
+git-filter-branch can be used to get rid of a subset of files,
+usually with some combination of `--index-filter` and
+`--subdirectory-filter`.  People expect the resulting repository to
+be smaller than the original, but you need a few more steps to
+actually make it smaller, because Git tries hard not to lose your
+objects until you tell it to.  First make sure that:
+
+* You really removed all variants of a filename, if a blob was moved
+  over its lifetime.  `git log --name-only --follow --all -- filename`
+  can help you find renames.
+
+* You really filtered all refs: use `--tag-name-filter cat -- --all`
+  when calling git-filter-branch.
+
+Then there are two ways to get a smaller repository.  A safer way is
+to clone, that keeps your original intact.
+
+* Clone it with `git clone file:///path/to/repo`.  The clone
+  will not have the removed objects.  See linkgit:git-clone[1].  (Note
+  that cloning with a plain path just hardlinks everything!)
+
+If you really don't want to clone it, for whatever reasons, check the
+following points instead (in this order).  This is a very destructive
+approach, so *make a backup* or go back to cloning it.  You have been
+warned.
+
+* Remove the original refs backed up by git-filter-branch: say `git
+  for-each-ref --format="%(refname)" refs/original/ | xargs -n 1 git
+  update-ref -d`.
+
+* Expire all reflogs with `git reflog expire --expire=now --all`.
+
+* Garbage collect all unreferenced objects with `git gc --prune=now`
+  (or if your git-gc is not new enough to support arguments to
+  `--prune`, use `git repack -ad; git prune` instead).
+
+NOTES
+-----
+
+git-filter-branch allows you to make complex shell-scripted rewrites
+of your Git history, but you probably don't need this flexibility if
+you're simply _removing unwanted data_ like large files or passwords.
+For those operations you may want to consider
+http://rtyley.github.io/bfg-repo-cleaner/[The BFG Repo-Cleaner],
+a JVM-based alternative to git-filter-branch, typically at least
+10-50x faster for those use-cases, and with quite different
+characteristics:
+
+* Any particular version of a file is cleaned exactly _once_. The BFG,
+  unlike git-filter-branch, does not give you the opportunity to
+  handle a file differently based on where or when it was committed
+  within your history. This constraint gives the core performance
+  benefit of The BFG, and is well-suited to the task of cleansing bad
+  data - you don't care _where_ the bad data is, you just want it
+  _gone_.
+
+* By default The BFG takes full advantage of multi-core machines,
+  cleansing commit file-trees in parallel. git-filter-branch cleans
+  commits sequentially (i.e. in a single-threaded manner), though it
+  _is_ possible to write filters that include their own parallelism,
+  in the scripts executed against each commit.
+
+* The http://rtyley.github.io/bfg-repo-cleaner/#examples[command options]
+  are much more restrictive than git-filter branch, and dedicated just
+  to the tasks of removing unwanted data- e.g:
+  `--strip-blobs-bigger-than 1M`.
+
+GIT
+---
+Part of the linkgit:git[1] suite