Support `arc bookmark` in Mercurial

Summary:
Branch in Mercurial means something else.
Hopefully users wouldn't be too confused.

Test Plan:
  $ arc help
  $ arc help branch
  $ arc help feature
  $ arc feature
  $ arc bookmark
  $ arc branch
  # In hg repo:
  $ arc feature
  $ arc feature new
  $ arc feature new

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2332

Differential Revision: https://secure.phabricator.com/D4753