Use repository mirroring fetches to move projects between storages

What does this MR do?

Modify the repository storage update code to use mirror fetches.

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

Is there anything we could be neglecting to copy using this method instead of the current one that relies on rsync?

Why was this MR needed?

In order to migrate repository storage update to Gitaly we have decided to rely on git operations instead of external commands (rsync) or filesystem operations. We can later migrate those git operations to Gitaly.

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#587 (closed)