Announce each operand on stderr before its passes

With per-operand walk/hash/update cycles, a multi-operand invocation
(e.g. scan /srv/*) showed pass totals that looked like the whole
run's: an operator watching operand 3 of 14 hash 300k files concluded
the other 20M files were being skipped. Print the operand path and
its position before each cycle.

scan.go

@@ -108,7 +108,13 @@ func syncScan(db *sql.DB, roots []string, workers int,
 ) (scanStats, error) {
 	var st scanStats
 
-	for _, root := range roots {
+	for i, root := range roots {
+		// Each operand runs its own walk/hash/update sequence, so
+		// announce it: without this, the per-operand pass totals look
+		// like the whole run's.
+		fmt.Fprintf(os.Stderr, "scan: %s (operand %d of %d)\n",
+			root, i+1, len(roots))
+
 		err := syncRoot(db, root, workers, oneFS, &st)
 		if err != nil {
 			return st, err