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

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

third_party/git/Documentation/RelNotes/1.7.11.5.txt

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+Git v1.7.11.5 Release Notes
+===========================
+
+Fixes since v1.7.11.4
+---------------------
+
+ * The Makefile rule to create assembly output (primarily for
+   debugging purposes) did not create it next to the source.
+
+ * The code to avoid mistaken attempt to add the object directory
+   itself as its own alternate could read beyond end of a string while
+   comparison.
+
+ * On some architectures, "block-sha1" did not compile correctly
+   when compilers inferred alignment guarantees from our source we
+   did not intend to make.
+
+ * When talking to a remote running ssh on IPv6 enabled host, whose
+   address is spelled as "[HOST]:PORT", we did not parse the address
+   correctly and failed to connect.
+
+ * git-blame.el (in compat/) have been updated to use Elisp more
+   correctly.
+
+ * "git checkout <branchname>" to come back from a detached HEAD state
+   incorrectly computed reachability of the detached HEAD, resulting
+   in unnecessary warnings.
+
+ * "git mergetool" did not support --tool-help option to give the list
+   of supported backends, like "git difftool" does.
+
+ * "git grep" stopped spawning an external "grep" long time ago, but a
+   duplicated test to check internal and external "grep" was left
+   behind.
+
+Also contains minor typofixes and documentation updates.