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
