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