Recheck if revision hasn't been already closed in commit parser

Summary:
If attaching a commit or checking if there are any changes takes nonzero time then the revision may be closed by someone else.

Cleaner solution would be to do it inside a transaction and mark the SELECT as FOR UPDATE but it would be blocking.

Test Plan: Patched $should_close to be true, reparsed an already closed commit.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3555