Fix some extra "changed the start time of this event" transactions

Summary: Ref T11326. Since we were missing an (int) cast here, the code ended up thinking that changing 12345 to "12345" was an edit. It isn't.

Test Plan: Created/edited events, no more extra "changed start time from X to the same X" transaction clutter.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11326

Differential Revision: https://secure.phabricator.com/D16773