If "test-tool bloom" is not fed a command, or if arguments are missing
for some commands, it will just segfault. Let's check argc and write a
friendlier usage message.
Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>