Mark revision as closed before attaching commit and loading changes

Summary:
D3555 stopped multiple commenting but it still attaches multiple diffs.

Save earlier to stop doing unnecessary work.

Test Plan: Reparsed the same commit twice at the same time.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3598