Make "Tags" and "Branches" custom fields work properly in Diffusion
Summary: Ref T8295 (vaguely related). This old method has no callsites.
Test Plan: Generated mail, saw "Tags" and "Branches".
Reviewers: fabe, joshuaspence
Reviewed By: fabe, joshuaspence
Maniphest Tasks: T8295
Differential Revision: https://secure.phabricator.com/D13000