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

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

third_party/git/contrib/hg-to-git/hg-to-git.txt

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+hg-to-git.py is able to convert a Mercurial repository into a git one,
+and preserves the branches in the process (unlike tailor)
+
+hg-to-git.py can probably be greatly improved (it's a rather crude
+combination of shell and python) but it does already work quite well for
+me. Features:
+	- supports incremental conversion
+	  (for keeping a git repo in sync with a hg one)
+        - supports hg branches
+        - converts hg tags
+
+Note that the git repository will be created 'in place' (at the same
+location as the source hg repo). You will have to manually remove the
+'.hg' directory after the conversion.
+
+Also note that the incremental conversion uses 'simple' hg changesets
+identifiers (ordinals, as opposed to SHA-1 ids), and since these ids
+are not stable across different repositories the hg-to-git.py state file
+is forever tied to one hg repository.
+
+Stelian Pop <stelian@popies.net>