Do not reload diff for merge request made from fork when target branch in fork is updated

What does this MR do?

The target branch of a merge request has to be a branch in the project for which the merge request is submitted. When a branch changes in a fork, it does not make sense to reload diffs of merge requests in the upstream project that use the same branch name as the target branch. Please note that it does make sense to reload diffs when the source branch changes.

Does this MR meet the acceptance criteria?

• Changelog entry added
• [ ] Documentation created/updated
• [ ] API support 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 #25263 (closed)