Clear caches before updating MR diffs

What does this MR do?

Basically it just swaps two callbacks with each other. However, if a merge request was updated the diff generation depended on the merge requests memoized values. In this case the oids for the branches. This meant that the diffs weren't properly updated.

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

What are the relevant issue numbers?

Fixes faulty behaviour introduced by: gitlab-org/gitlab-ce!15857