Reload source MRs memoization after diffs creation

What does this MR do?

Addresses https://gitlab.com/gitlab-org/gitlab-ce/issues/42668.

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16690 introduces this flaw because it calls merge_requests_for_source_branch before every other method in RefreshService. This method memoizes MRs, which then, gets updated by reload_merge_requests (creating MergeRequestDiff's). In upcoming calls like MergeRequest#commit_shas, the memoized version returns commit_shas of the staled MergeRequestDiff.

To address that, we update the memoized Array, refetching the MRs after getting diffs 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
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/42668