Add user parameter to differential.createrevision
Summary: arc incorrectly passes a "user" parameter to differential.createrevision (long ago, we respected it, I think). After D4191 this fatals. Provide a stub call until the next version bump.
Test Plan: inspection
Reviewers: vrana, btrahan
Reviewed By: btrahan
CC: aran
Differential Revision: https://secure.phabricator.com/D4220