Prevent "Differential Revision: ..." from counting as a mention in commit messages

Summary:
Ref T13290. Ref T13291. Now that a full URI is a "mention", the full URI in "Differential Revision: ..." also triggers a mention.

Stop it from doing that, since these mentions are silly/redundant/unintended.

The API here is also slightly odd; simplify it a little bit to get rid of doing "append" with "get + append + set".

Test Plan: Used bin/repository reparse --publish to republish commits with "Differential Revision: ..." and verified that the revision PHID was properly dropped from the mention list.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13291, T13290

Differential Revision: https://secure.phabricator.com/D20544