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

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

third_party/git/Documentation/RelNotes/1.6.4.2.txt

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+GIT v1.6.4.2 Release Notes
+==========================
+
+Fixes since v1.6.4.1
+--------------------
+
+* --date=relative output between 1 and 5 years ago rounded the number of
+    years when saying X years Y months ago, instead of rounding it down.
+
+* "git add -p" did not handle changes in executable bits correctly
+  (a regression around 1.6.3).
+
+* "git apply" did not honor GNU diff's convention to mark the creation/deletion
+  event with UNIX epoch timestamp on missing side.
+
+* "git checkout" incorrectly removed files in a directory pointed by a
+  symbolic link during a branch switch that replaces a directory with
+  a symbolic link.
+
+* "git clean -d -f" happily descended into a subdirectory that is managed by a
+  separate git repository.  It now requires two -f options for safety.
+
+* "git fetch/push" over http transports had two rather grave bugs.
+
+* "git format-patch --cover-letter" did not prepare the cover letter file
+  for use with non-ASCII strings when there are the series contributors with
+  non-ASCII names.
+
+* "git pull origin branch" and "git fetch origin && git merge origin/branch"
+  left different merge messages in the resulting commit.
+
+Other minor documentation updates are included.