Raw diff and rendered diff produce different results

Zendesk: https://gitlab.zendesk.com/agent/tickets/18305

The raw diff endpoint (merge_requests/####.diff) produces a diff with lots more changes than the regular merge request diffs tab. Maybe this is simply a lack of understanding about the differences between the two, but it seems like a bug.

Possibly related, and interesting, is that merge_requests/####.patch yields yet a third result.

I used !3388 (merged) as a test case.

.diff URL

Regular diff view

.patch URL

nothing is generated

cc/ @zj @brodock Can you explain or see what's going on here, please?