Separate revision updates into two separate comments

Summary:
Ref T2222. Instead of writing one comment which performs both a diff update and adds a comment, write two comments, one for each action. These will translate directly into ApplicationTransactions writes.

This has a small impact on the UX: these updates now render in two rows, instead of one. After T2222, they'll automerge back together.

{F111909}

Test Plan: Updated a revision.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2222

Differential Revision: https://secure.phabricator.com/D8200