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

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+Git v2.2 Release Notes
+======================
+
+Updates since v2.1
+------------------
+
+Ports
+
+ * Building on older MacOS X systems automatically sets
+   the necessary NO_APPLE_COMMON_CRYPTO build-time option.
+
+ * Building with NO_PTHREADS has been resurrected.
+
+ * Compilation options have been updated a bit to better support the
+   z/OS port.
+
+
+UI, Workflows & Features
+
+ * "git archive" learned to filter what gets archived with a pathspec.
+
+ * "git config --edit --global" starts from a skeletal per-user
+   configuration file contents, instead of a total blank, when the
+   user does not already have any global config.  This immediately
+   reduces the need to later ask "Have you forgotten to set
+   core.user?", and we can add more to the template as we gain
+   more experience.
+
+ * "git stash list -p" used to be almost always a no-op because each
+   stash entry is represented as a merge commit.  It learned to show
+   the difference between the base commit version and the working tree
+   version, which is in line with what "git stash show" gives.
+
+ * Sometimes users want to report a bug they experience on their
+   repository, but they are not at liberty to share the contents of
+   the repository.  "fast-export" was taught an "--anonymize" option
+   to replace blob contents, names of people, paths and log
+   messages with bland and simple strings to help them.
+
+ * "git difftool" learned an option to stop feeding paths to the
+   diff backend when it exits with a non-zero status.
+
+ * "git grep" learned to paint (or not paint) partial matches on
+   context lines when showing "grep -C<num>" output in color.
+
+ * "log --date=iso" uses a slight variant of the ISO 8601 format that is
+   more human readable.  A new "--date=iso-strict" option gives
+   datetime output that conforms more strictly.
+
+ * The logic "git prune" uses is more resilient against various corner
+   cases.
+
+ * A broken reimplementation of Git could write an invalid index that
+   records both stage #0 and higher-stage entries for the same path.
+   We now notice and reject such an index, as there is no sensible
+   fallback (we do not know if the broken tool wanted to resolve and
+   forgot to remove the higher-stage entries, or if it wanted to unresolve
+   and forgot to remove the stage #0 entry).
+
+ * The temporary files "git mergetool" uses are renamed to avoid too
+   many dots in them (e.g. a temporary file for "hello.c" used to be
+   named e.g. "hello.BASE.4321.c" but now uses underscore instead,
+   e.g. "hello_BASE_4321.c", to allow us to have multiple variants).
+
+ * The temporary files "git mergetool" uses can be placed in a newly
+   created temporary directory, instead of the current directory, by
+   setting the mergetool.writeToTemp configuration variable.
+
+ * "git mergetool" understands "--tool bc" now, as version 4 of
+   BeyondCompare can be driven the same way as its version 3 and it
+   feels awkward to say "--tool bc3" to run version 4.
+
+ * The "pre-receive" and "post-receive" hooks are no longer required
+   to consume their input fully (not following this requirement used
+   to result in intermittent errors in "git push").
+
+ * The pretty-format specifier "%d", which expands to " (tagname)"
+   for a tagged commit, gained a cousin "%D" that just gives the
+   "tagname" without frills.
+
+ * "git push" learned "--signed" push, that allows a push (i.e.
+   request to update the refs on the other side to point at a new
+   history, together with the transmission of necessary objects) to be
+   signed, so that it can be verified and audited, using the GPG
+   signature of the person who pushed, that the tips of branches at a
+   public repository really point the commits the pusher wanted to,
+   without having to "trust" the server.
+
+ * "git interpret-trailers" is a new filter to programmatically edit
+   the tail end of the commit log messages, e.g. "Signed-off-by:".
+
+ * "git help everyday" shows the "Everyday Git in 20 commands or so"
+   document, whose contents have been updated to match more modern
+   Git practice.
+
+ * On the "git svn" front, work progresses to reduce memory consumption and
+   to improve handling of mergeinfo.
+
+
+Performance, Internal Implementation, Development Support etc.
+
+ * The API to manipulate the "refs" has been restructured to make it
+   more transactional, with the eventual goal to allow all-or-none
+   atomic updates and migrating the storage to something other than
+   the traditional filesystem based one (e.g. databases).
+
+ * The lockfile API and its users have been cleaned up.
+
+ * We no longer attempt to keep track of individual dependencies to
+   the header files in the build procedure, relying instead on automated
+   dependency generation support from modern compilers.
+
+ * In tests, we have been using NOT_{MINGW,CYGWIN} test prerequisites
+   long before negated prerequisites e.g. !MINGW were invented.
+   The former has been converted to the latter to avoid confusion.
+
+ * Optimized looking up a remote's configuration in a repository with very many
+   remotes defined.
+
+ * There are cases where you lock and open to write a file, close it
+   to show the updated contents to an external processes, and then have
+   to update the file again while still holding the lock; now the
+   lockfile API has support for such an access pattern.
+
+ * The API to allocate the structure to keep track of commit
+   decoration has been updated to make it less cumbersome to use.
+
+ * An in-core caching layer to let us avoid reading the same
+   configuration files several times has been added.  A few commands
+   have been converted to use this subsystem.
+
+ * Various code paths have been cleaned up and simplified by using
+   the "strbuf", "starts_with()", and "skip_prefix()" APIs more.
+
+ * A few codepaths that died when large blobs that would not fit in
+   core are involved in their operation have been taught to punt
+   instead, by e.g. marking a too-large blob as not to be diffed.
+
+ * A few more code paths in "commit" and "checkout" have been taught
+   to repopulate the cache-tree in the index, to help speed up later
+   "write-tree" (used in "commit") and "diff-index --cached" (used in
+   "status").
+
+ * A common programming mistake to assign the same short option name
+   to two separate options is detected by the parse_options() API to help
+   developers.
+
+ * The code path to write out the packed-refs file has been optimized,
+   which especially matters in a repository with a large number of
+   refs.
+
+ * The check to see if a ref $F can be created by making sure no
+   existing ref has $F/ as its prefix has been optimized, which
+   especially matters in a repository with a large number of existing
+   refs.
+
+ * "git fsck" was taught to check the contents of tag objects a bit more.
+
+ * "git hash-object" was taught a "--literally" option to help
+   debugging.
+
+ * When running a required clean filter, we do not have to mmap the
+   original before feeding the filter.  Instead, stream the file
+   contents directly to the filter and process its output.
+
+ * The scripts in the test suite can be run with the "-x" option to show
+   a shell-trace of each command they run.
+
+ * The "run-command" API learned to manage the argv and environment
+   arrays for child process, alleviating the need for the callers to
+   allocate and deallocate them.
+
+ * Some people use AsciiDoctor, instead of AsciiDoc, to format our
+   documentation set; the documentation has been adjusted to be usable
+   by both, as AsciiDoctor is pickier than AsciiDoc about its input
+   mark-up.
+
+
+Also contains various documentation updates and code clean-ups.
+
+
+Fixes since v2.1
+----------------
+
+Unless otherwise noted, all the fixes since v2.1 in the maintenance
+track are contained in this release (see the maintenance releases'
+notes for details).
+
+ * "git log --pretty/format=" with an empty format string did not
+   mean the more obvious "No output whatsoever" but "Use default
+   format", which was counterintuitive.
+
+ * "git -c section.var command" and "git -c section.var= command"
+   should pass the configuration value differently (the former should be a
+   boolean true, the latter should be an empty string).
+
+ * Applying a patch not generated by Git in a subdirectory used to
+   check for whitespace breakage using the attributes of incorrect
+   paths. Also whitespace checks were performed even for paths
+   excluded via the "git apply --exclude=<path>" mechanism.
+
+ * "git bundle create" with a date-range specification was meant to
+   exclude tags outside the range, but it didn't.
+
+ * "git add x" where x used to be a directory and is now a
+   symbolic link to a directory misbehaved.
+
+ * The prompt script checked the $GIT_DIR/ref/stash file to see if there
+   is a stash, which was a no-no.
+
+ * Pack-protocol documentation had a minor typo.
+
+ * "git checkout -m" did not switch to another branch while carrying
+   the local changes forward when a path was deleted from the index.
+
+ * "git daemon" (with NO_IPV6 build configuration) used to incorrectly
+   use the hostname even when gethostbyname() reported that the given
+   hostname is not found.
+   (merge 107efbe rs/daemon-fixes later to maint).
+
+ * With sufficiently long refnames, "git fast-import" could have
+   overflowed an on-stack buffer.
+
+ * After "pack-refs --prune" packed refs at the top-level, it failed
+   to prune them.
+
+ * Progress output from "git gc --auto" was visible in "git fetch -q".
+
+ * We used to pass -1000 to poll(2), expecting it to also mean "no
+   timeout", which should be spelled as -1.
+
+ * "git rebase" documentation was unclear that it is required to
+   specify on what <upstream> the rebase is to be done when telling it
+   to first check out <branch>.
+   (merge 95c6826 so/rebase-doc later to maint).
+
+ * "git push" over HTTP transport had an artificial limit on the number of
+   refs that can be pushed, imposed by the command line length.
+   (merge 26be19b jk/send-pack-many-refspecs later to maint).
+
+ * When receiving an invalid pack stream that records the same object
+   twice, multiple threads got confused due to a race.
+   (merge ab791dd jk/index-pack-threading-races later to maint).
+
+ * An attempt to remove the entire tree in the "git fast-import" input
+   stream caused it to misbehave.
+   (merge 2668d69 mb/fast-import-delete-root later to maint).
+
+ * Reachability check (used in "git prune" and friends) did not add a
+   detached HEAD as a starting point to traverse objects still in use.
+   (merge c40fdd0 mk/reachable-protect-detached-head later to maint).
+
+ * "git config --add section.var val" when section.var already has an
+   empty-string value used to lose the empty-string value.
+   (merge c1063be ta/config-add-to-empty-or-true-fix later to maint).
+
+ * "git fsck" failed to report that it found corrupt objects via its
+   exit status in some cases.
+   (merge 30d1038 jk/fsck-exit-code-fix later to maint).
+
+ * Use of the "--verbose" option used to break "git branch --merged".
+   (merge 12994dd jk/maint-branch-verbose-merged later to maint).
+
+ * Some MUAs mangle a line in a message that begins with "From " to
+   ">From " when writing to a mailbox file, and feeding such an input
+   to "git am" used to lose such a line.
+   (merge 85de86a jk/mbox-from-line later to maint).
+
+ * "rev-parse --verify --quiet $name" is meant to quietly exit with a
+   non-zero status when $name is not a valid object name, but still
+   gave error messages in some cases.
+
+ * A handful of C source files have been updated to include
+   "git-compat-util.h" as the first thing, to conform better to our
+   coding guidelines.
+   (merge 1c4b660 da/include-compat-util-first-in-c later to maint).
+
+ * The t7004 test, which tried to run Git with small stack space, has been
+   updated to use a bit larger stack to avoid false breakage on some
+   platforms.
+   (merge b9a1907 sk/tag-contains-wo-recursion later to maint).
+
+ * A few documentation pages had example sections marked up not quite
+   correctly, which passed AsciiDoc but failed with AsciiDoctor.
+   (merge c30c43c bc/asciidoc-pretty-formats-fix later to maint).
+   (merge f8a48af bc/asciidoc later to maint).
+
+ * "gitweb" used deprecated CGI::startfrom, which was removed from
+   CGI.pm as of 4.04; use CGI::start_from instead.
+   (merge 4750f4b rm/gitweb-start-form later to maint).
+
+ * Newer versions of 'meld' break the auto-detection we use to see if
+   they are new enough to support the `--output` option.
+   (merge b12d045 da/mergetool-meld later to maint).
+
+ * "git pack-objects" forgot to disable the codepath to generate the
+   object reachability bitmap when it needs to split the resulting
+   pack.
+   (merge 2113471 jk/pack-objects-no-bitmap-when-splitting later to maint).
+
+ * The code to use cache-tree trusted the on-disk data too much and
+   fell into an infinite loop upon seeing an incorrectly recorded
+   index file.
+   (merge 729dbbd jk/cache-tree-protect-from-broken-libgit2 later to maint).
+
+ * "git fetch" into a repository where branch B was deleted earlier,
+   back when it had reflog enabled, and then branch B/C is fetched
+   into it without reflog enabled, which is arguably an unlikely
+   corner case, unnecessarily failed.
+   (merge aae828b jk/fetch-reflog-df-conflict later to maint).
+
+ * "git log --first-parent -L..." used to crash.
+   (merge a8787c5 tm/line-log-first-parent later to maint).