Merge requests use wrong commits for diffing images
Created by: davidfraser
Merge requests show "old" and "new" images side by side in diffs. However, the "old" image comes from the parent of the HEAD of the merge request, so any changes that didn't happen in the final commit are not shown correctly.
See https://gitlab.com/davidfraser/gitlab-test/merge_requests/1/diffs as an example