remote-hg: use notes to keep track of Hg revisions

remote-hg: use notes to keep track of Hg revisions

Keep track of Mercurial revisions as Git notes under the 'refs/notes/hg'
ref.  This way, the user can easily see which Mercurial revision
corresponds to certain Git commit.

Unfortunately, there's no way to efficiently update the notes after
doing an export (push), so they'll have to be updated when importing
(fetching).

Signed-off-by: Felipe Contreras <redacted>
Signed-off-by: Junio C Hamano <redacted>