Don't re-mention users for comment edits

Summary:
Ref T11035. This only fixes half of the issue: comment editing has been fixed, but normal transactions which edit things like descriptions haven't yet.

The normal edits aren't fixed because the "oldValues" are populated too late. The code should start working once they get populated sooner, but I don't want to jump the gun on that since it'll probably have some spooky effects. I have some other transaction changes coming down the pipe which should provide a better context for testing "oldValue" population order.

Test Plan:

• Mentioned @dog in a comment.
• Removed @dog as a subscriber.
• Edited the comment, adding some unrelated text at the end (e.g., fixing a typo).
  • Before change: @dog re-added as subscriber.
  • After change: no re-add.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11035

Differential Revision: https://secure.phabricator.com/D16108