Fix issues mentioned but not closed for JIRA

What does this MR do?

When using an external issue tracker, we would show issues that were closed by an MR as both:

• Accepting this merge request will close issue X.
• Issue X is mentioned but will not be closed.

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

I feel a little uncertain about overriding ExternalIssue#hash, but it seems the simplest solution.

Why was this MR needed?

The bug was really dumb.

Screenshots (if relevant)

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 https://gitlab.com/gitlab-org/gitlab-ce/issues/26028.