Mark reviewers as "commented" when they leave a comment

Summary:
Ref T2222. This requires one new trick:

  - When merging edge transactions which both add/update an edge, the Editor gets to control how the edge data is merged.

Specifically, we pick the "strongest" state to keep, so "accept + comment" leaves you with an accept instead of a comment.

Test Plan: Accepted, commented on, and comment + accepted revisions. Added some debugging dumps to verify that the merging was getting hit and working correctly.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2222

Differential Revision: https://secure.phabricator.com/D8340