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 (closed).