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

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

third_party/git/Documentation/RelNotes/2.6.4.txt

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+Git v2.6.4 Release Notes
+========================
+
+Fixes since v2.6.3
+------------------
+
+ * The "configure" script did not test for -lpthread correctly, which
+   upset some linkers.
+
+ * Add support for talking http/https over socks proxy.
+
+ * Portability fix for Windows, which may rewrite $SHELL variable using
+   non-POSIX paths.
+
+ * We now consistently allow all hooks to ignore their standard input,
+   rather than having git complain of SIGPIPE.
+
+ * Fix shell quoting in contrib script.
+
+ * Test portability fix for a topic in v2.6.1.
+
+ * Allow tilde-expansion in some http config variables.
+
+ * Give a useful special case "diff/show --word-diff-regex=." as an
+   example in the documentation.
+
+ * Fix for a corner case in filter-branch.
+
+ * Make git-p4 work on a detached head.
+
+ * Documentation clarification for "check-ignore" without "--verbose".
+
+ * Just like the working tree is cleaned up when the user cancelled
+   submission in P4Submit.applyCommit(), clean up the mess if "p4
+   submit" fails.
+
+ * Having a leftover .idx file without corresponding .pack file in
+   the repository hurts performance; "git gc" learned to prune them.
+
+ * The code to prepare the working tree side of temporary directory
+   for the "dir-diff" feature forgot that symbolic links need not be
+   copied (or symlinked) to the temporary area, as the code already
+   special cases and overwrites them.  Besides, it was wrong to try
+   computing the object name of the target of symbolic link, which may
+   not even exist or may be a directory.
+
+ * There was no way to defeat a configured rebase.autostash variable
+   from the command line, as "git rebase --no-autostash" was missing.
+
+ * Allow "git interpret-trailers" to run outside of a Git repository.
+
+ * Produce correct "dirty" marker for shell prompts, even when we
+   are on an orphan or an unborn branch.
+
+ * Some corner cases have been fixed in string-matching done in "git
+   status".
+
+ * Apple's common crypto implementation of SHA1_Update() does not take
+   more than 4GB at a time, and we now have a compile-time workaround
+   for it.
+
+Also contains typofixes, documentation updates and trivial code
+clean-ups.