For protocol 30, the sender uses the new idev_node() hashfile

routine to keep track of which item is the first one in a particular
hard-link cluster.  It then abbreviates the sending of any follow-on
items in the cluster, reducing transfer bytes.  It also omits the
sending of any dev+inode data to the receiver, saving even more xfer
bytes (since the receiver can associate the entries based on the the
group's index number that is sent when abbreviating an entry).