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

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

third_party/git/Documentation/RelNotes/2.3.8.txt

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+Git v2.3.8 Release Notes
+========================
+
+Fixes since v2.3.7
+------------------
+
+ * The usual "git diff" when seeing a file turning into a directory
+   showed a patchset to remove the file and create all files in the
+   directory, but "git diff --no-index" simply refused to work.  Also,
+   when asked to compare a file and a directory, imitate POSIX "diff"
+   and compare the file with the file with the same name in the
+   directory, instead of refusing to run.
+
+ * The default $HOME/.gitconfig file created upon "git config --global"
+   that edits it had incorrectly spelled user.name and user.email
+   entries in it.
+
+ * "git commit --date=now" or anything that relies on approxidate lost
+   the daylight-saving-time offset.
+
+Also contains typofixes, documentation updates and trivial code
+clean-ups.