MR discussion tab gives a 500 when commented-on commit from fork no longer exists

On staging, this MR fails to load: https://staging.gitlab.com/gitlab-org/gitlab-ce/merge_requests/7313 / https://sentry.gitlap.com/gitlab/staginggitlabcom/issues/37225/

That's because there is a commit comment on the forked repo, but the forked repo doesn't exist: https://staging.gitlab.com/gkats/gitlab-ce

We shouldn't rely on the presence of a commit in the repo here, although this particular case is very 'edge'. Cases I tested:

1. Fork is private, upstream is public. Result: commit comment remains visible.
2. Fork has repository disabled (after adding commit). Result: commit comment remains visible.
3. Fork is deleted. Result: we close the MR and don't try to show the comment.

So this can only really happen if something breaks somehow.