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

Merge commit '1b593e1ea4d2af0f6444d9a7788d5d99abd6fde5' as 'third_party/git'

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+Git v2.1 Release Notes
+======================
+
+Backward compatibility notes
+----------------------------
+
+ * The default value we give to the environment variable LESS has been
+   changed from "FRSX" to "FRX", losing "S" (chop long lines instead
+   of wrapping).  Existing users who prefer not to see line-wrapped
+   output may want to set
+
+     $ git config core.pager "less -S"
+
+   to restore the traditional behaviour.  It is expected that people
+   find output from most subcommands easier to read with the new
+   default, except for "blame" which tends to produce really long
+   lines.  To override the new default only for "git blame", you can
+   do this:
+
+     $ git config pager.blame "less -S"
+
+ * A few disused directories in contrib/ have been retired.
+
+
+Updates since v2.0
+------------------
+
+UI, Workflows & Features
+
+ * Since the very beginning of Git, we gave the LESS environment a
+   default value "FRSX" when we spawn "less" as the pager.  "S" (chop
+   long lines instead of wrapping) has been removed from this default
+   set of options, because it is more or less a personal taste thing,
+   as opposed to the others that have good justifications (i.e. "R" is
+   very much justified because many kinds of output we produce are
+   colored and "FX" is justified because output we produce is often
+   shorter than a page).
+
+ * The logic and data used to compute the display width needed for
+   UTF-8 strings have been updated to match Unicode 7.0 better.
+
+ * HTTP-based transports learned to better propagate the error messages from
+   the webserver to the client coming over the HTTP transport.
+
+ * The completion script for bash (in contrib/) has been updated to
+   better handle aliases that define a complex sequence of commands.
+
+ * The "core.preloadindex" configuration variable is enabled by default,
+   allowing modern platforms to take advantage of their
+   multiple cores.
+
+ * "git clone" applies the "if cloning from a local disk, physically
+   copy the repository using hardlinks, unless otherwise told not to with
+   --no-local" optimization when the url.*.insteadOf mechanism rewrites a
+   remote-repository "git clone $URL" into a
+   clone from a local disk.
+
+ * "git commit --date=<date>" option learned more
+   timestamp formats, including "--date=now".
+
+ * The `core.commentChar` configuration variable is used to specify a
+   custom comment character (other than the default "#") for
+   the commit message editor.  This can be set to `auto` to attempt to
+   choose a different character that does not conflict with any that
+   already starts a line in the message being edited, for cases like
+   "git commit --amend".
+
+ * "git format-patch" learned --signature-file=<file> to add the contents
+   of a file as a signature to the mail message it produces.
+
+ * "git grep" learned the grep.fullname configuration variable to force
+   "--full-name" to be the default.  This may cause regressions for
+   scripted users who do not expect this new behaviour.
+
+ * "git imap-send" learned to ask the credential helper for auth
+   material.
+
+ * "git log" and friends now understand the value "auto" for the
+   "log.decorate" configuration variable to enable the "--decorate"
+   option automatically when the output is sent to tty.
+
+ * "git merge" without an argument, even when there is an upstream
+   defined for the current branch, refused to run until
+   merge.defaultToUpstream is set to true.  Flip the default of that
+   configuration variable to true.
+
+ * "git mergetool" learned to drive the vimdiff3 backend.
+
+ * mergetool.prompt used to default to 'true', always asking "do you
+   really want to run the tool on this path?".  The default has been
+   changed to 'false'.  However, the prompt will still appear if
+   mergetool used its autodetection system to guess which tool to use.
+   Users who explicitly specify or configure a tool will no longer see
+   the prompt by default.
+
+   Strictly speaking, this is a backward incompatible change and
+   users need to explicitly set the variable to 'true' if they want
+   to be prompted to confirm running the tool on each path.
+
+ * "git replace" learned the "--edit" subcommand to create a
+   replacement by editing an existing object.
+
+ * "git replace" learned a "--graft" option to rewrite the parents of a
+   commit.
+
+ * "git send-email" learned "--to-cover" and "--cc-cover" options, to
+   tell it to copy To: and Cc: headers found in the first input file
+   when emitting later input files.
+
+ * "git svn" learned to cope with malformed timestamps with only one
+   digit in the hour part, e.g. 2014-01-07T5:01:02.048176Z, emitted
+   by some broken subversion server implementations.
+
+ * "git tag" when editing the tag message shows the name of the tag
+   being edited as a comment in the editor.
+
+ * "git tag" learned to pay attention to "tag.sort" configuration, to
+   be used as the default sort order when no --sort=<value> option
+   is given.
+
+ * A new "git verify-commit" command, to check GPG signatures in signed
+   commits, in a way similar to "git verify-tag" is used to check
+   signed tags, was added.
+
+
+Performance, Internal Implementation, etc.
+
+ * Build procedure for 'subtree' (in contrib/) has been cleaned up.
+
+ * Support for the profile-feedback build, which has
+   bit-rotted for quite a while, has been updated.
+
+ * An experimental format to use two files (the base file and
+   incremental changes relative to it) to represent the index has been
+   introduced; this may reduce I/O cost of rewriting a large index
+   when only small part of the working tree changes.
+
+ * Effort to shrink the size of patches Windows folks maintain on top
+   by upstreaming them continues.  More tests that are not applicable
+   to the Windows environment are identified and either skipped or
+   made more portable.
+
+ * Eradication of "test $condition -a $condition" from our scripts
+   continues.
+
+ * The `core.deltabasecachelimit` used to default to 16 MiB , but this
+   proved to be too small, and has been bumped to 96 MiB.
+
+ * "git blame" has been optimized greatly by reorganising the data
+   structure that is used to keep track of the work to be done.
+
+ * "git diff" that compares 3-or-more trees (e.g. parents and the
+   result of a merge) has been optimized.
+
+ * The API to update/delete references are being converted to handle
+   updates to multiple references in a transactional way.  As an
+   example, "update-ref --stdin [-z]" has been updated to use this
+   API.
+
+ * skip_prefix() and strip_suffix() API functions are used a lot more
+   widely throughout the codebase now.
+
+ * Parts of the test scripts can be skipped by using a range notation,
+   e.g. "sh t1234-test.sh --run='1-4 6 8-'" to omit test piece 5 and 7
+   and run everything else.
+
+
+Also contains various documentation updates and code clean-ups.
+
+
+Fixes since v2.0
+----------------
+
+Unless otherwise noted, all the fixes since v2.0 in the maintenance
+track are contained in this release (see the maintenance releases'
+notes for details).
+
+ * We used to unconditionally disable the pager in the pager process
+   we spawn to feed out output, but that prevented people who want to
+   run "less" within "less" from doing so.
+   (merge c0459ca je/pager-do-not-recurse later to maint).
+
+ * Tools that read diagnostic output in our standard error stream do
+   not want to see terminal control sequence (e.g. erase-to-eol).
+   Detect them by checking if the standard error stream is connected
+   to a tty.
+   (merge 38de156 mn/sideband-no-ansi later to maint).
+
+ * Mishandling of patterns in .gitignore that have trailing SPs quoted
+   with backslashes (e.g. ones that end with "\ ") has been
+   corrected.
+   (merge 97c1364be6b pb/trim-trailing-spaces later to maint).
+
+ * Reworded the error message given upon a failure to open an existing
+   loose object file due to e.g. permission issues; it was reported as
+   the object being corrupt, but that is not quite true.
+   (merge d6c8a05 jk/report-fail-to-read-objects-better later to maint).
+
+ * "git log -2master" is a common typo that shows two commits starting
+   from whichever random branch that is not 'master' that happens to
+   be checked out currently.
+   (merge e3fa568 jc/revision-dash-count-parsing later to maint).
+
+ * Code to avoid adding the same alternate object store twice was
+   subtly broken for a long time, but nobody seems to have noticed.
+   (merge 80b4785 rs/fix-alt-odb-path-comparison later to maint).
+   (merge 539e750 ek/alt-odb-entry-fix later to maint).
+
+ * The "%<(10,trunc)%s" pretty format specifier in the log family of
+   commands is used to truncate the string to a given length (e.g. 10
+   in the example) with padding to column-align the output, but did
+   not take into account that number of bytes and number of display
+   columns are different.
+   (merge 7d50987 as/pretty-truncate later to maint).
+
+ * "%G" (nothing after G) is an invalid pretty format specifier, but
+   the parser did not notice it as garbage.
+   (merge 958b2eb jk/pretty-G-format-fixes later to maint).
+
+ * A handful of code paths had to read the commit object more than
+   once when showing header fields that are usually not parsed.  The
+   internal data structure to keep track of the contents of the commit
+   object has been updated to reduce the need for this double-reading,
+   and to allow the caller find the length of the object.
+   (merge 218aa3a jk/commit-buffer-length later to maint).
+
+ * The "mailmap.file" configuration option did not support tilde
+   expansion (i.e. ~user/path and ~/path).
+   (merge 9352fd5 ow/config-mailmap-pathname later to maint).
+
+ * The completion scripts (in contrib/) did not know about quite a few
+   options that are common between "git merge" and "git pull", and a
+   couple of options unique to "git merge".
+   (merge 8fee872 jk/complete-merge-pull later to maint).
+
+ * The unix-domain socket used by the sample credential cache daemon
+   tried to unlink an existing stale one at a wrong path, if the path
+   to the socket was given as an overlong path that does not fit in
+   the sun_path member of the sockaddr_un structure.
+   (merge 2869b3e rs/fix-unlink-unix-socket later to maint).
+
+ * An ancient rewrite passed a wrong pointer to a curl library
+   function in a rarely used code path.
+   (merge 479eaa8 ah/fix-http-push later to maint).
+
+ * "--ignore-space-change" option of "git apply" ignored the spaces
+   at the beginning of lines too aggressively, which is inconsistent
+   with the option of the same name that "diff" and "git diff" have.
+   (merge 14d3bb4 jc/apply-ignore-whitespace later to maint).
+
+ * "git blame" miscounted the number of columns needed to show localized
+   timestamps, resulting in a jaggy left-side-edge for the source code
+   lines in its output.
+   (merge dd75553 jx/blame-align-relative-time later to maint).
+
+ * "git blame" assigned the blame to the copy in the working-tree if
+   the repository is set to core.autocrlf=input and the file used CRLF
+   line endings.
+   (merge 4d4813a bc/blame-crlf-test later to maint).
+
+ * "git clone -b brefs/tags/bar" would have mistakenly thought we were
+   following a single tag, even though it was a name of the branch,
+   because it incorrectly used strstr().
+   (merge 60a5f5f jc/fix-clone-single-starting-at-a-tag later to maint).
+
+ * "git commit --allow-empty-message -C $commit" did not work when the
+   commit did not have any log message.
+   (merge 076cbd6 jk/commit-C-pick-empty later to maint).
+
+ * "git diff --find-copies-harder" sometimes pretended as if the mode
+   bits have changed for paths that are marked with the assume-unchanged
+   bit.
+   (merge 5304810 jk/diff-files-assume-unchanged later to maint).
+
+ * "filter-branch" left an empty single-parent commit that results when
+   all parents of a merge commit get mapped to the same commit, even
+   under "--prune-empty".
+   (merge 79bc4ef cb/filter-branch-prune-empty-degenerate-merges later to maint).
+
+ * "git format-patch" did not enforce the rule that the "--follow"
+   option from the log/diff family of commands must be used with
+   exactly one pathspec.
+   (merge dd63f16 jk/diff-follow-must-take-one-pathspec later to maint).
+
+ * "git gc --auto" was recently changed to run in the background to
+   give control back early to the end-user sitting in front of the
+   terminal, but it forgot that housekeeping involving reflogs should
+   be done without other processes competing for accesses to the refs.
+   (merge 62aad18 nd/daemonize-gc later to maint).
+
+ * "git grep -O" to show the lines that hit in the pager did not work
+   well with case insensitive search.  We now spawn "less" with its
+   "-I" option when it is used as the pager (which is the default).
+   (merge f7febbe sk/spawn-less-case-insensitively-from-grep-O-i later to maint).
+
+ * We used to disable threaded "git index-pack" on platforms without
+   thread-safe pread(); use a different workaround for such
+   platforms to allow threaded "git index-pack".
+   (merge 3953949 nd/index-pack-one-fd-per-thread later to maint).
+
+ * The error reporting from "git index-pack" has been improved to
+   distinguish missing objects from type errors.
+   (merge 77583e7 jk/index-pack-report-missing later to maint).
+
+ * "log --show-signature" incorrectly decided the color to paint a
+   mergetag that was and was not correctly validated.
+   (merge 42c55ce mg/fix-log-mergetag-color later to maint).
+
+ * "log --show-signature" did not pay attention to the "--graph" option.
+   (merge cf3983d zk/log-graph-showsig later to maint).
+
+ * "git mailinfo" used to read beyond the ends of header strings while
+   parsing an incoming e-mail message to extract the patch.
+   (merge b1a013d rs/mailinfo-header-cmp later to maint).
+
+ * On a case insensitive filesystem, merge-recursive incorrectly
+   deleted the file that is to be renamed to a name that is the same
+   except for case differences.
+   (merge baa37bf dt/merge-recursive-case-insensitive later to maint).
+
+ * Merging changes into a file that ends in an incomplete line made the
+   last line into a complete one, even when the other branch did not
+   change anything around the end of file.
+   (merge ba31180 mk/merge-incomplete-files later to maint).
+
+ * "git pack-objects" unnecessarily copied the previous contents when
+   extending the hashtable, even though it will populate the table
+   from scratch anyway.
+   (merge fb79947 rs/pack-objects-no-unnecessary-realloc later to maint).
+
+ * Recent updates to "git repack" started to duplicate objects that
+   are in packfiles marked with the .keep flag into the new packfile by
+   mistake.
+   (merge d078d85 jk/repack-pack-keep-objects later to maint).
+
+ * "git rerere forget" did not work well when merge.conflictstyle
+   was set to a non-default value.
+   (merge de3d8bb fc/rerere-conflict-style later to maint).
+
+ * "git remote rm" and "git remote prune" can involve removing many
+   refs at once, which is not a very efficient thing to do when very
+   many refs exist in the packed-refs file.
+   (merge e6bea66 jl/remote-rm-prune later to maint).
+
+ * "git log --exclude=<glob> --all | git shortlog" worked as expected,
+   but "git shortlog --exclude=<glob> --all", which is supposed to be
+   identical to the above pipeline, was not accepted at the command
+   line argument parser level.
+   (merge eb07774 jc/shortlog-ref-exclude later to maint).
+
+ * The autostash mode of "git rebase -i" did not restore the dirty
+   working tree state if the user aborted the interactive rebase by
+   emptying the insn sheet.
+   (merge ddb5432 rr/rebase-autostash-fix later to maint).
+
+ * "git rebase --fork-point" did not filter out patch-identical
+   commits correctly.
+
+ * During "git rebase --merge", a conflicted patch could not be
+   skipped with "--skip" if the next one also conflicted.
+   (merge 95104c7 bc/fix-rebase-merge-skip later to maint).
+
+ * "git show -s" (i.e. show log message only) used to incorrectly emit
+   an extra blank line after a merge commit.
+   (merge ad2f725 mk/show-s-no-extra-blank-line-for-merges later to maint).
+
+ * "git status", even though it is a read-only operation, tries to
+   update the index with refreshed lstat(2) info to optimize future
+   accesses to the working tree opportunistically, but this could
+   race with a "read-write" operation that modifies the index while it
+   is running.  Detect such a race and avoid overwriting the index.
+   (merge 426ddee ym/fix-opportunistic-index-update-race later to maint).
+
+ * "git status" (and "git commit") behaved as if changes in a modified
+   submodule are not there if submodule.*.ignore configuration is set,
+   which was misleading.  The configuration is only to unclutter diff
+   output during the course of development, and not to hide
+   changes in the "status" output to cause the users forget to commit
+   them.
+   (merge c215d3d jl/status-added-submodule-is-never-ignored later to maint).
+
+ * Documentation for "git submodule sync" forgot to say that the subcommand
+   can take the "--recursive" option.
+   (merge 9393ae7 mc/doc-submodule-sync-recurse later to maint).
+
+ * "git update-index --cacheinfo" in 2.0 release crashed on a
+   malformed command line.
+   (merge c8e1ee4 jc/rev-parse-argh-dashed-multi-words later to maint).
+
+ * The mode to run tests with HTTP server tests disabled was broken.
+   (merge afa53fe na/no-http-test-in-the-middle later to maint).