Don't use rugged in Repository#refs_hash

What does this MR do?

Remove the need for Rugged in Gitlab::Git::Repository#refs_hash.

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?

gitaly#880 (closed)