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

Merge commit '1b593e1ea4' as 'third_party/git'

third_party/git/Documentation/RelNotes/1.5.6.4.txt

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+GIT v1.5.6.4 Release Notes
+==========================
+
+Fixes since v1.5.6.3
+--------------------
+
+* Various commands could overflow its internal buffer on a platform
+  with small PATH_MAX value in a repository that has contents with
+  long pathnames.
+
+* There wasn't a way to make --pretty=format:%<> specifiers to honor
+  .mailmap name rewriting for authors and committers.  Now you can with
+  %aN and %cN.
+
+* Bash completion wasted too many cycles; this has been optimized to be
+  usable again.
+
+* Bash completion lost ref part when completing something like "git show
+  pu:Makefile".
+
+* "git-cvsserver" did not clean up its temporary working area after annotate
+  request.
+
+* "git-daemon" called syslog() from its signal handler, which was a
+  no-no.
+
+* "git-fetch" into an empty repository used to remind that the fetch will
+   be huge by saying "no common commits", but this was an unnecessary
+   noise; it is already known by the user anyway.
+
+* "git-http-fetch" would have segfaulted when pack idx file retrieved
+  from the other side was corrupt.
+
+* "git-index-pack" used too much memory when dealing with a deep delta chain.
+
+* "git-mailinfo" (hence "git-am") did not correctly handle in-body [PATCH]
+  line to override the commit title taken from the mail Subject header.
+
+* "git-rebase -i -p" lost parents that are not involved in the history
+  being rewritten.
+
+* "git-rm" lost track of where the index file was when GIT_DIR was
+  specified as a relative path.
+
+* "git-rev-list --quiet" was not quiet as advertised.
+
+Contains other various documentation fixes.