Transactions - don't let objects mention themselves.

Summary: Fixes T6059.

Test Plan: Made a comment on TX mentioning TX and TX+1. TX did not get a "mentioned" transaction while TX+1 did.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T6059

Differential Revision: https://secure.phabricator.com/D10464