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