Clarify logging when replication start hits a persist-only remote

A remote configured with threads=0 persists ref updates to
ReplicationTasksStorage for another node to pick up, but never
increments the push task count. Running `replication start --wait`
against such a remote therefore printed "No replication scheduled.",
which reads as if nothing happened even though the refs were
persisted.

Change-Id: I769e97c4b1f1b87267e3fc24ac054218a460dbbe