Praefect's track-repository/track-repositories commands cannot track repos using 'cluster' paths

Praefect's track-repository and track-repositories subcommands will use the relative path of the repo being tracked as both relative path and replica path. For repos using the old @hashed paths this is fine, but with @cluster paths the association from the path used by rails to Praefect's internal path is lost.

These commands are now unable to re-integrate repositories into a Gitaly Cluster.

/cc @mjwood