Close and do not reload MR diffs when source branch is deleted

What does this MR do?

Automatically closes and keep last valid MR diff when source branch ref is deleted.

Why was this MR needed?

Further explained on https://gitlab.com/gitlab-org/gitlab-ce/issues/37775

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #37775 (closed)