Extend TransactionCommentQuery for Differential

Summary: Ref T2009. Ref T1460. Replace hard-coded garbage with a real Query-layer query.

Test Plan: Submitted inline comments in Differential.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T2009, T1460

Differential Revision: https://secure.phabricator.com/D12027