Add external MR diffs to `gitlab-backup`

Problem

External MR diffs on disk are not included in backups by gitlab-backup.

I also assumed external MR diffs were included in backups. I mentioned it in backup docs a couple times.

I notice that the CNG backup-utility includes external MR diffs https://gitlab.com/gitlab-org/build/CNG/-/blob/7d2ad33db89b4ed0421c809b7364274cfd8ff6c4/gitlab-toolbox/scripts/bin/backup-utility#L19. So this gap only affects gitlab-backup when storing diffs on disk.

I thought perhaps MR diff data might be automatically regenerated, but this issue makes it sounds like it is not.

(Note: gitlab-backup excludes all object storage)

Proposal

Include external MR diffs on disk in backups by gitlab-backup.

Implementation guide

Follow the example of !121142 (merged).

An important note: Only merge_request_diffs which are stored_externally: true have associated files.