Migration manager stores migration keys with the snapshot relative path

partition: Introduce migration framework (!7198 - merged) introduced tooling to author repository migrations in Gitaly. It stores in the embedded database the migration status of a repository under a key like m/<relative-path>.

Migration keys are created and deleted by the transactions that create and delete repositories. The key is constructed in the RPC handler but we're not accounting for the relative path rewriting that occurs with transactions. This results in us storing the migration key under for the wrong relative path like m/+gitaly/staging/3251461931/snapshots/1/@hashed/aa/cb/aacb454dc92a610d6cade67013901ca777e1c224db4e1f4371769be2b448d4a5.git.

We should handle the relative path rewriting and point the migration key to the original relative path.