Force Differential draft uniqueness
Summary: Ref T1191. A couple of installs have hit issues with this table, so clean it up before adjustment adds a unique key to it.
Test Plan: Dropped key, added duplicate rows, ran patch, got cleanup, ran adjust to get the key back.
Reviewed By: btrahan
Maniphest Tasks: T1191
Differential Revision: https://secure.phabricator.com/D10799