Always mark revisions as "updated" when users comment on them

Summary: See T773 and the explanatory inline comment.

Test Plan: Made no-action comments and comments that did something (reject, plan
changes) to revisions. Saw them always jump to the top of the action list.

Reviewers: jungejason, simpkins, btrahan

Reviewed By: jungejason

CC: aran, jungejason

Maniphest Tasks: T773

Differential Revision: https://secure.phabricator.com/D1386