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

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

third_party/git/Documentation/RelNotes/2.3.4.txt

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+Git v2.3.4 Release Notes
+========================
+
+Fixes since v2.3.3
+------------------
+
+ * The 'color.status.unmerged' configuration was not described.
+
+ * "git log --decorate" did not reset colors correctly around the
+   branch names.
+
+ * "git -C '' subcmd" refused to work in the current directory, unlike
+   "cd ''" which silently behaves as a no-op.
+
+ * "git imap-send" learned to optionally talk with an IMAP server via
+   libcURL; because there is no other option when Git is built with
+   NO_OPENSSL option, use that codepath by default under such
+   configuration.
+
+ * A workaround for certain build of GPG that triggered false breakage
+   in a test has been added.
+
+ * "git rebase -i" recently started to include the number of
+   commits in the insn sheet to be processed, but on a platform
+   that prepends leading whitespaces to "wc -l" output, the numbers
+   are shown with extra whitespaces that aren't necessary.
+
+ * We did not parse username followed by literal IPv6 address in SSH
+   transport URLs, e.g. ssh://user@[2001:db8::1]:22/repo.git
+   correctly.
+
+Also contains typofixes, documentation updates and trivial code clean-ups.