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

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

third_party/git/Documentation/RelNotes/1.6.2.2.txt

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+GIT v1.6.2.2 Release Notes
+==========================
+
+Fixes since v1.6.2.1
+--------------------
+
+* A longstanding confusing description of what --pickaxe option of
+  git-diff does has been clarified in the documentation.
+
+* "git-blame -S" did not quite work near the commits that were given
+  on the command line correctly.
+
+* "git diff --pickaxe-regexp" did not count overlapping matches
+  correctly.
+
+* "git diff" did not feed files in work-tree representation to external
+  diff and textconv.
+
+* "git-fetch" in a repository that was not cloned from anywhere said
+  it cannot find 'origin', which was hard to understand for new people.
+
+* "git-format-patch --numbered-files --stdout" did not have to die of
+  incompatible options; it now simply ignores --numbered-files as no files
+  are produced anyway.
+
+* "git-ls-files --deleted" did not work well with GIT_DIR&GIT_WORK_TREE.
+
+* "git-read-tree A B C..." without -m option has been broken for a long
+  time.
+
+* git-send-email ignored --in-reply-to when --no-thread was given.
+
+* 'git-submodule add' did not tolerate extra slashes and ./ in the path it
+  accepted from the command line; it now is more lenient.
+
+* git-svn misbehaved when the project contained a path that began with
+  two dashes.
+
+* import-zips script (in contrib) did not compute the common directory
+  prefix correctly.
+
+* miscompilation of negated enum constants by old gcc (2.9) affected the
+  codepaths to spawn subprocesses.
+
+Many small documentation updates are included as well.