Show MR for squashed and merge commits
We currently show which MR a commit came from: https://docs.gitlab.com/ee/user/project/merge_requests/#find-the-merge-request-that-introduced-a-change
However, this doesn't apply in two cases:
- When the MR is squashed.
- For the merge commit itself.
The merge commit does, by default, include a reference to the MR in its message.
We already have enough information to do 2, and I believe it wouldn't be too hard to do 1. We wouldn't be able to backfill this, though, just do it for MRs going forward.
There is no change in the design for showing related MRs for all kinds of commits.