R1:1eff4fdca332

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…