Restore "Branch" and "changes since last update" fields to Differential mail

Summary: Ref T418. Fixes T4642. The "changes since last update" and "branch" fields got dropped; restore them in a general, field-driven way.

Test Plan:

• Created a revision, got relevant sections in mail.
• Commented on a revision, got relevant sections in mail.
• Updated a revision, got relevant sections in mail.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: spicyj, epriestley

Maniphest Tasks: T418, T4642

Differential Revision: https://secure.phabricator.com/D8657