Differential - make sure not to return change type header if we're not top level

Summary: Followup to D6924. Fixes T3824.

Test Plan: deleted a file in a diff. was able to view file content without JS errors

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, aran

Maniphest Tasks: T3824

Differential Revision: https://secure.phabricator.com/D6963