Fix bug with batched gitaly ref deletion duplicates

What does this MR do and why?

Fix bug with batched gitaly ref deletion duplicates

If there are duplicates sent in a single Git command we get an error. This de-duplication works around this. We could also consider de-duplicating in the queue but this is tricky to do without race conditions and would also require a migration. Without the uniq fix this new test fails with:

  1) BatchedGitRefUpdates::ProjectCleanupService#execute handles duplicates
     Failure/Error: raise Gitlab::Git::CommandError, e

     Gitlab::Git::CommandError:
       13:unable to prepare: reference has been updated multiple times within a transaction.

Related to:

• #428381 (closed)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.