Merge request comments made in the context of a specific commit no longer specify which commit they were created on

Both of these comments were created on a specific commit in the MR, by going to the Commits tab and viewing just the diffs of a specific commit:

The byline of the first one should say "on commit [sha]", and the second one, which is outdated, should say "on an outdated change in commit [sha]", per https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12148/diffs#afe5105460af0b757065118d37a6f786eddafb7f.

This issue is similar to https://gitlab.com/gitlab-org/gitlab-ce/issues/53950, but different, because while that one concerns comments created on the commit page for a commit that happens to be part of an MR (/:namespace/:project/commit/:sha), this one concerns comments created on a commit in the context of the MR (/:namespace/:project/merge_requests/:iid/diffs?commit_id=:sha).

/cc @fatihacet @andr3

Background

A fix for this consists of two parts:

• sending commit ID for comments that are made in the context of a commit on a merge request
• displaying the commit ID for diff comments