Separate field for branch in revision

Summary:
The main purpose of this change is to allow selecting the branch by
triple-click.
Plus it is not perfectly clear that the text in brackets means branch.

Test Plan: Display revision.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1585