Transactions - don't bother checking for fancy transactions on comment create

Summary: Only necessary for edits, only bother if the comment version is greater than 1. Ref T6690. This is another way to fix T6690 -- this check will never run since you can't edit a conpherence comment -- but the fix already applied should happen too to future proof Conpherence.

Test Plan: made a comment on a diff - success. edited the comment and mentions were generated.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6690

Differential Revision: https://secure.phabricator.com/D10928