Link squashed commits using the changelog API

What does this MR do?

When generating changelogs using the API, we now link to merge requests for squashed commits. Prior to this commit, when a merge request was squashed into a target branch, we weren't able to link back to that merge request in a generated changelog.

See gitlab-com/gl-infra/delivery#1551 (closed) for more information.

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides