More Gitaly cleanup: fetch_ref, allow disk access blocks

What does this MR do?

• removes the gitaly_fetch_ref feature flag
• adds allow_disk_access blocks inside gitaly_migrate blocks so that we don't get exceptions if we have to turn off any feature flags
• refactors a call from MergeRequestParser to the meant-to-be-private Gitlab::Git::Repository#fetch_ref method

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

Why was this MR needed?

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
• Conforms to the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes gitaly#1295 (closed)