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

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

third_party/git/Documentation/RelNotes/2.4.5.txt

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+Git v2.4.5 Release Notes
+========================
+
+Fixes since v2.4.4
+------------------
+
+ * The setup code used to die when core.bare and core.worktree are set
+   inconsistently, even for commands that do not need working tree.
+
+ * There was a dead code that used to handle "git pull --tags" and
+   show special-cased error message, which was made irrelevant when
+   the semantics of the option changed back in Git 1.9 days.
+
+ * "color.diff.plain" was a misnomer; give it 'color.diff.context' as
+   a more logical synonym.
+
+ * The configuration reader/writer uses mmap(2) interface to access
+   the files; when we find a directory, it barfed with "Out of memory?".
+
+ * Recent "git prune" traverses young unreachable objects to safekeep
+   old objects in the reachability chain from them, which sometimes
+   showed unnecessary error messages that are alarming.
+
+ * "git rebase -i" fired post-rewrite hook when it shouldn't (namely,
+   when it was told to stop sequencing with 'exec' insn).
+
+Also contains typofixes, documentation updates and trivial code
+clean-ups.