Let fetch_ref pull from Gitaly instead of from disk

What does this MR do?

Adds a feature flag to let Gitlab::Git::Repository#fetch_ref pull from Gitaly instead of from disk.

I found a 'creative' use of fetch_ref that would get much less efficient because of this (it was fetching from the repo into itself) so I refactored some code to be able to remove that one call site of fetch_ref.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Gitaly all the things.

Screenshots (if relevant)

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

What are the relevant issue numbers?

Closes gitaly#585 (closed)