R1:53ffaaa889ec

Load buildables in DiffViewController

Summary: Fixes T9128. I missed this when converting other controllers to look in Harbormaster for lint/unit data.

Test Plan: Copy/pasted a diff successfully.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9128

Differential Revision: https://secure.phabricator.com/D13865