Fix diff comments on legacy MRs

## What does this MR do?

Allow diff commenting on MRs without complete diff refs. (Commenting would work before, but the JSON response would fail, so you'd only see your comment after a refresh.)

## Are there points in the code the reviewer needs to double check?

Is this really all I needed to do? I feel like there must be more to it 😊

## Why was this MR needed?

It's a regression!

## What are the relevant issue numbers?

Closes #21211.

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !6029