Draft: backup: Fix dangling repo comparison

This MR fixes some issues introduced in !6579 (merged).

The problems were first brought to our attention in this Slack thread and were blocking a security release. The root cause of the failed spec was the new restore logic introduced in the MR linked above.

To unblock the security release, a revert MR was merged into the Gitaly security mirror. Once the release is published, the security mirror will be merged back into this repository. Once that happens, this MR can be merged and will include the original commits plus the fixes.