Add support to 'arc browse' to turn a commit-ish into a revision.

Summary:
Now you can say 'arc browse HEAD' -- or even just 'arc browse' -- and
it will figure out what phabricator revision HEAD is for, and open up
that revision in a browser.

With this, it's very practical to do

  arc diff
  arc browse   # then add a comment to the arc revision.

Replaces D11877

Test Plan:
Ran 'arc browse' on this very commit, and had it open up the new
review in my browser.

Then ran
   arc browse rGWA1f38772bd953d68be8c1ba82588e0c4893806f67
and it showed that commit, without printing any other error messages.

Reviewers: benkomalo

Reviewed By: benkomalo

Differential Revision: http://phabricator.khanacademy.org/D13980