Remove "--allow-empty-message"

Summary: @alex has git 1.7.0.4 which doesn't have this flag. We don't actually need it: we always provide a commit message when calling this method. Remove the flag for compatibility, leaving a note in case we bump into this in the future.

Test Plan: N/A

Reviewers: btrahan, vrana, alex

Reviewed By: alex

CC: aran

Differential Revision: https://secure.phabricator.com/D5926