Gently move some listeners into DiffChangesetList

Summary: Ref T12616. Put these listeners in DiffChangesetList so the wake/sleep properly for Quicksand.

Test Plan:

• Added some logging.
• With quicksand, moved between diffs.
• Saw "load" and "show more" fire exactly once on each page, with the correct changeset list listener.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12616

Differential Revision: https://secure.phabricator.com/D17841