Add Branch and Arcanist Project CustomFields

Summary:
Ref T2222. These are pretty straightforward.

For these fields and a few others, the existing code shows the value for the "current/manual" diff (i.e., the diff selected in the diff selection table), not the "active" diff (i.e., the most recent diff attached to the revision). I'm going to drop that for now (always showing the most recent diff instead) and then reevaluate it once we're switched over. In 95% of cases these are the same, anyway.

Test Plan: Looked at fields; this diff changes nothing on its own.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2222

Differential Revision: https://secure.phabricator.com/D8359