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

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

third_party/git/Documentation/RelNotes/1.8.5.5.txt

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+Git v1.8.5.5 Release Notes
+==========================
+
+Fixes since v1.8.5.4
+--------------------
+
+ * The pathspec matching code, while comparing two trees (e.g. "git
+   diff A B -- path1 path2") was too aggressive and failed to match
+   some paths when multiple pathspecs were involved.
+
+ * "git repack --max-pack-size=8g" stopped being parsed correctly when
+   the command was reimplemented in C.
+
+ * A recent update to "git send-email" broke platforms where
+   /etc/ssl/certs/ directory exists but cannot be used as SSL_ca_path
+   (e.g. Fedora rawhide).
+
+ * A handful of bugs around interpreting $branch@{upstream} notation
+   and its lookalike, when $branch part has interesting characters,
+   e.g. "@", and ":", have been fixed.
+
+ * "git clone" would fail to clone from a repository that has a ref
+   directly under "refs/", e.g. "refs/stash", because different
+   validation paths do different things on such a refname.  Loosen the
+   client side's validation to allow such a ref.
+
+ * "git log --left-right A...B" lost the "leftness" of commits
+   reachable from A when A is a tag as a side effect of a recent
+   bugfix.  This is a regression in 1.8.4.x series.
+
+ * "git merge-base --octopus" used to leave cleaning up suboptimal
+   result to the caller, but now it does the clean-up itself.
+
+ * "git mv A B/", when B does not exist as a directory, should error
+   out, but it didn't.
+
+Also contains typofixes, documentation updates and trivial code clean-ups.