Fix patch 131 for db with lots of revisions

Summary:
The old version of this loads all differential revisions at once, but that much
can't all be loaded into memory when there are close to 500,000 revisions. This
diff splits up loading the revisions.

Test Plan: Ran this to run the migration in our install

Reviewers: jungejason, epriestley

Reviewed By: epriestley

CC: aran

Differential Revision: https://secure.phabricator.com/D2243