Add Branch and Arcanist Project CustomFields
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.
Reviewed By: btrahan
Maniphest Tasks: T2222
Differential Revision: https://secure.phabricator.com/D8359