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

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

third_party/git/Documentation/RelNotes/1.6.5.4.txt

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+Git v1.6.5.4 Release Notes
+==========================
+
+Fixes since v1.6.5.3
+--------------------
+
+ * "git help" (without argument) used to check if you are in a directory
+   under git control. There was no breakage in behaviour per-se, but this
+   was unnecessary.
+
+ * "git prune-packed" gave progress output even when its standard error is
+   not connected to a terminal; this caused cron jobs that run it to
+   produce crufts.
+
+ * "git pack-objects --all-progress" is an option to ask progress output
+   from write-object phase _if_ progress output were to be produced, and
+   shouldn't have forced the progress output.
+
+ * "git apply -p<n> --directory=<elsewhere>" did not work well for a
+   non-default value of n.
+
+ * "git merge foo HEAD" was misparsed as an old-style invocation of the
+   command and produced a confusing error message.  As it does not specify
+   any other branch to merge, it shouldn't be mistaken as such.  We will
+   remove the old style "git merge <message> HEAD <commit>..."  syntax in
+   future versions, but not in this release,
+
+ * "git merge -m <message> <branch>..." added the standard merge message
+   on its own after user-supplied message, which should have overridden the
+   standard one.
+
+Other minor documentation updates are included.