This will be used by the frontend to show the banner of the overflow

MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/64195

Changelog: added

Add spec and factory for multiple diffs

If merge_request_diff for some reason isn't contained in
@merge_request_diffs, we will see a TypeError

Changelog: fixed

When group/project/merge_requests/iid/diffs?diff_head=true
is visited, then the diff which compared against HEAD
will be shown

Partially addresses #47424.