Allow differing relative paths in ReplicateRepository (15.5)

Praefect has an artificial constraint where the configuration is validated not to have multiple storages hosted by a given Gitaly. This is not a real requirement. It should be possible to host multiple storages in a virtual storage on a given Gitaly node. It's not generally a good idea though given a single node doesn't offer redundancy. There's shouldn't be a reason to fail hard on this, so let's remove the check.

8c7cee30 is cherry-picked as well as the tests will fail without it since ReplicateRepository did not work in Praefect with an existing repository before it. It's nice to include it anyway here as it does away with another recurring problem in ReplicateRepositroy and migrating repositories across shards.

Backport of !5076 (merged)