Add CustomField to Diffusion and add a "branches" field

Summary:
Ref T4588. Request from @zeeg. Adds a "BRANCHES" field to commit emails, so the branches the commit appears on are shown.

I've implemented this with CustomField, but in a very light way.

Test Plan: Used `scripts/repository/reparse.php --herald` to generate mail, got a BRANCHES section where applicable.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: aran, epriestley, zeeg

Maniphest Tasks: T4588

Differential Revision: https://secure.phabricator.com/D8501