Recent MergeRequestDiff changes introduced a TypeError

@jacobvosmaer: @alfredo1 hit a TypeError using the GDK and one of its seeded projects (twitter/typeahead-js).

I narrowed it down to MergeRequest#last_commit returning nil because its associated MergeRequestDiff is now showing as having no commits for some reason. I checked a previous revision of MergeRequestDiff and this error was likely introduced by 1764e1b7.

If you visit /twitter/typeahead-js/merge_requests/1 with a seeded GDK install you can reproduce it. It seems to only happen on this one merge request so it's possible we're just seeding it incorrectly for the recent changes.