Display commit ID for commit diff discussion on merge request

What does this MR do?

Displays started discussion on commit ... instead of on the diff in merge requests for comments made on the diff of commits.

What are the relevant issue numbers?

part of https://gitlab.com/gitlab-org/gitlab-ce/issues/53950

Screenshots

Before	After

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.