Improve mercurial 'arc land' perf by avoiding updates

Summary:
Mercurial 'arc land --hold' was taking 90+ seconds on our large
repository.  Since most of arc land doesn't require any particular working
directory, I've changed the mercurial logic to avoid all updates except for
two: the one prior to finding the revision (only applies if the user specified
--branch), and the one at the end to leave the user in a good state.

Also got rid of a 'hg outgoing' call when phases are supported. Also changed
the hg-subversion detection to just look for .hg/svn instead of running 'hg
svn info', which was taking 4 seconds.

Now arc land takes about 50 seconds. Still much worse than git's 25 seconds.
One big hot spot is in the two 'hg rebase' calls, which account for 25 seconds
(versus 11 seconds of git).

Test Plan:
Tested arc land with mercurial and git. Tested with and without the --branch
options.

Reviewers: epriestley, bos, sid0, dschleimer

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5014