Handle case where deployment ref no longer exists

What does this MR do?

In 8.9, we didn't create keep-around refs for deployments. So it's possible that someone created a deployment (say, for testing), and then deleted the branch and all other references to that commit. That commit could then get GCed, and trying to view MRs on 8.11+ will show a 500. See https://gitlab.com/gitlab-org/gitlab-ce/issues/22655#note_16575020 for more details.

Are there points in the code the reviewer needs to double check?

This seems to be the minimal change to fix the issue; always worth checking though!

Why was this MR needed?

If someone created a deployment on 8.9, then deleted all references to the commit for that deployment, we will throw an exception when checking if the deployment includes a commit.

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #22655 (closed).