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

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

third_party/git/Documentation/RelNotes/2.3.7.txt

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+Git v2.3.7 Release Notes
+========================
+
+Fixes since v2.3.6
+------------------
+
+ * An earlier update to the parser that disects a URL broke an
+   address, followed by a colon, followed by an empty string (instead
+   of the port number), e.g. ssh://example.com:/path/to/repo.
+
+ * The completion script (in contrib/) contaminated global namespace
+   and clobbered on a shell variable $x.
+
+ * The "git push --signed" protocol extension did not limit what the
+   "nonce" that is a server-chosen string can contain or how long it
+   can be, which was unnecessarily lax.  Limit both the length and the
+   alphabet to a reasonably small space that can still have enough
+   entropy.
+
+Also contains typofixes, documentation updates and trivial code
+clean-ups.