remote prune: print the list of pruned branches

remote prune: print the list of pruned branches

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>