prune-packed: advanced progress even for non-existing fan-out directories

prune-packed: advanced progress even for non-existing fan-out directories

A progress indicator is used to count through the 256 object fan-out
directories while unused object files are removed. (However, it becomes
visible only if this process takes long enough.)

Previously, display_progress() was only called if object files were
actually removed. But if directories towards the end (fd/, fe/, ff/) did
not exist, this could leave a strange line

   Removing duplicate objects:  99% (255/256), done.

in the terminal instead of the expected "100% (256/256)".

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>