The multi-pack-index builtin has a very simple command-line
interface. Instead of simply reporting usage, give the user a
hint to why the arguments failed.
Reported-by: Eric Sunshine <sunshine@sunshineco.com> Signed-off-by: Derrick Stolee <dstolee@microsoft.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>