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

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

third_party/git/Documentation/RelNotes/1.5.2.1.txt

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+GIT v1.5.2.1 Release Notes
+==========================
+
+Fixes since v1.5.2
+------------------
+
+* Bugfixes
+
+  - Temporary files that are used when invoking external diff
+    programs did not tolerate a long TMPDIR.
+
+  - git-daemon did not notice when it could not write into its
+    pid file.
+
+  - git-status did not honor core.excludesFile configuration like
+    git-add did.
+
+  - git-annotate did not work from a subdirectory while
+    git-blame did.
+
+  - git-cvsserver should have disabled access to a repository
+    with "gitcvs.pserver.enabled = false" set even when
+    "gitcvs.enabled = true" was set at the same time.  It
+    didn't.
+
+  - git-cvsimport did not work correctly in a repository with
+    its branch heads were packed with pack-refs.
+
+  - ident unexpansion to squash "$Id: xxx $" that is in the
+    repository copy removed incorrect number of bytes.
+
+  - git-svn misbehaved when the subversion repository did not
+    provide MD5 checksums for files.
+
+  - git rebase (and git am) misbehaved on commits that have '\n'
+    (literally backslash and en, not a linefeed) in the title.
+
+  - code to decode base85 used in binary patches had one error
+    return codepath wrong.
+
+  - RFC2047 Q encoding output by git-format-patch used '_' for a
+    space, which is not understood by some programs.  It uses =20
+    which is safer.
+
+  - git-fastimport --import-marks was broken; fixed.
+
+  - A lot of documentation updates, clarifications and fixes.