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

Subscribers: epriestley

Maniphest Tasks: T8295

Differential Revision: https://secure.phabricator.com/D13000