"-" abbreviation is handy for "cherry-pick" like "checkout" and "merge".
It's also good for uniformity that a "-" stands as
the name of the previous branch where a branch name is
accepted and it could not mean any other things like stdin.
Signed-off-by: Hiroshige Umino <hiroshige88@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>