Transactions - fix pagination bug

Summary: Fixes T6694. Ref T4712. Turns out the logic here was slightly incorrect; we don't want to use the id of the last thing we hid but rather the first thing we show. I had garbage test data ("asdsadsadsa", etc) I guess so I didn't notice this.

Test Plan: made a new task where user a and user b alternated 3 comments each, cooperatively numbering them from 1 - 20. as both users, showed older transactions. pre-patch the issue described in T6694 occurred and post patch I saw the entire counting sequence.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T4712, T6694

Differential Revision: https://secure.phabricator.com/D10933