Fix issue where new transaction comments can be spread across multiple transactions

Summary: Ref T2222. We need this `clone` when constructing the new multi-comments in Differential, or we get double-comments internally. This shows up as emails with double comment text.

Test Plan: Sent some "Accept + comment" emails, only one comment in the body.

Reviewers: chad, btrahan

Reviewed By: chad

CC: aran

Maniphest Tasks: T2222

Differential Revision: https://secure.phabricator.com/D8206