universe: don't spew the diff

At times spew can choke on a data structure like our smt tree. Not sure
why this didn't really care before, but does now. Either way, allows a
test to proceed.

universe/auto_syncer.go

@@ -202,8 +202,8 @@ func (f *FederationEnvoy) syncServerState(ctx context.Context,
 	}
 
 	// If we synced anything from the server, then we'll log that here.
-	log.Infof("Synced new Universe leaves from server=%v, diff=%v",
-		spew.Sdump(addr), spew.Sdump(diff))
+	log.Infof("Synced new Universe leaves from server=%v, diff_size=%v",
+		spew.Sdump(addr), len(diff))
 
 	// Log a new sync event in the background now that we know we were able
 	// to contract the remote server.