Smooth "no effect" detection in Maniphest for @mentions

Summary:
Ref T2217. Since we aren't actually using subscriptions yet, the "transactions have no effect" detection can trigger for `@mention`s of users who are already CC'd on a task.

Be more conservative about generating a CC transaction.

Test Plan: See screenshot.

Reviewers: btrahan, chad

Reviewed By: chad

CC: chad, aran

Maniphest Tasks: T2217

Differential Revision: https://secure.phabricator.com/D7095