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