Add branch queries to differential.query

Summary: This enables some improvements in D1478. Allow revisons to be queried
by the branch which they appear on.

Test Plan: Queried revisions by branch. Ran "arc which" branch queries in SVN
and Mercurial.

Reviewers: btrahan, cpiro, jungejason

Reviewed By: btrahan

CC: aran, epriestley

Maniphest Tasks: T787

Differential Revision: https://secure.phabricator.com/D1479