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.

README.md

@@ -209,6 +209,15 @@ operand before starting the next:
 3.  **update** — apply the operand's insertions, updates, and
     deletions to the database in a single transaction.
 
+Because every operand runs its own three passes with its own totals,
+`scan` announces each operand on stderr before starting it, so a
+multi-operand invocation (e.g. a shell glob) is not mistaken for a
+nearly-finished run:
+
+```
+scan: /srv/media (operand 3 of 14)
+```
+
 Rules for the walk:
 
 -   Only regular files. Skip directories, symlinks (do not follow,