Reroute batch blobs to single blob RPC

What does this MR do?

Reroute the batch endpoint to Gitlab::Git::Blob.find so we don't have to write an extra endpoint for now. This will keep the N + 1 calls as bad as they were, which sadly is expected behaviour.

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