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