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