R1:cf6f7446ce3f

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…