Migrate ReplicateRepository to use SSHUploadPackWithSidechannel

Our internal fetches are currently using SSHUploadPack on the remote side to fetch the data. This RPC is using gRPC messages to send over data, which is creating a lot of memory allocations due to the way gRPC is structured, which can cause excessive load on the host. We have a new SSHUploadPackWithSidechannel RPC which avoids this issue by using a sidechannel, so we should likely migrate internal fetches to use that RPC instead.