This command is really too quiet which make it unconfortable to use.
Also implement a --dry-run option, in place of the original -n one, to
list stale tracking branches that will be pruned, but do not actually
prune them.
Add a test case for --dry-run.
Signed-off-by: Olivier Marin <dkr@freesurf.fr> Signed-off-by: Junio C Hamano <gitster@pobox.com>