WIP: Improve GitHub import branches performance

This will allow us to:

• Creating MR diffs was timing out recently, so we couldn't even resume the import. Having this should help with performance.
• Fix current MRs that have no actual branch associated and are throwing 500s
• Fix MRs that have been updated upstream (so the diff has changed) but not locally
• Performance should be improved as we won't be fetching the branch per MR

After this is merged we can then remove all (non-required) branches.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/36288

• 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

What are the relevant issue numbers?