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

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

third_party/git/Documentation/RelNotes/1.7.5.4.txt

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+Git v1.7.5.4 Release Notes
+==========================
+
+Fixes since v1.7.5.3
+--------------------
+
+ * The single-key mode of "git add -p" was easily fooled into thinking
+   that it was told to add everything ('a') when up-arrow was pressed by
+   mistake.
+
+ * Setting a git command that uses custom configuration via "-c var=val"
+   as an alias caused a crash due to a realloc(3) failure.
+
+ * "git diff -C -C" used to disable the rename detection entirely when
+   there are too many copy candidate paths in the tree; now it falls
+   back to "-C" when doing so would keep the copy candidate paths
+   under the rename detection limit.
+
+ * "git rerere" did not diagnose a corrupt MERGE_RR file in some cases.
+
+And other minor fixes and documentation updates.