Fix Import/Export issue to do with fork merge requests

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/36666

Importing forked MRs resulted in an error to do with not exporting the *_branch_sha attributes properly. Also, using project.repository.branch_exists? uses a cached version of the branch names that we can't use in case MRs point to the same branch, as that will lead to an error.

• 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?