Fix arguments error on Import/Export fetch_ref method

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/39541

• Adds missing named parameters https://gitlab.com/gitlab-org/gitlab-ce/issues/39541#note_46265831 and repository (it was using repository path in the past)
• Adds unit test
• Updates integration spec to have a forked MR that also fails on these conditions

• 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