Correct a case where a single-hunk diff may incorrectly be identified as multi-hunk by the Scope engine

Summary: See PHI985. The layers above this may return array() to mean "one hunk with a line-1 offset". Accept either array() or array(1 => ...) to engage the scope engine.

Test Plan: See PHI985.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20363