Bandage an issue with the file tree view for unusual versus diffs

Summary: See comment inline. We should fix this properly but it goes faily deep so I just stopped the bleeding for now. It's OK if we end up with a silly-looking file tree view for bizarre edge cases.

Test Plan: Created a problem diff as described, verified it no longer threw.

Reviewers: vrana, btrahan

Reviewed By: vrana

CC: aran, Avish

Maniphest Tasks: T1702

Differential Revision: https://secure.phabricator.com/D3373