Maintain the pool is broken because of missing refmap

When Garbage collection is performed on the source project of a pool repository, it will try to fetch new objects from the source project into the pool. This RPC is basically a no-op right now as there's on refmap set. This results in Git not fetching any more objects.

To fix the refmap can be hard coded to the fetch command that's executed. This would make the fetch update the pool with new objects and with that reduce the duplication in a network of forks.

This issue is inconvenient, but not a blocker for GA for git object deduplication