restored --head support
Summary: the call to setHeadCommit() was accidentally placed in a if ($background) {} block, which was removed in 54bea94. This adds the removed call.
Test Plan: used --head, worked as expected
Reviewers: epriestley, Blessed Reviewers
Reviewed By: epriestley, Blessed Reviewers
Subscribers: epriestley, Korvin
Differential Revision: https://secure.phabricator.com/D10447