commit-graph: show "unexpected subcommand" error
Bring the "commit-graph" command in line with the error output and
general pattern in cmd_multi_pack_index().
Let's test for that output, and also cover the same potential bug as
was fixed in the multi-pack-index command in
88617d11f9d (multi-pack-index: fix potential segfault without
sub-command, 2021-07-19).
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>