parse-options: option to let --git-completion-helper show negative form

parse-options: option to let --git-completion-helper show negative form

When 7fb6aefd2a (Merge branch 'nd/parseopt-completion' - 2018-03-14)
is merged, the completion for negative form is left out because the
series is alread long and it could be done in a follow up series. This
is it.

--git-completion-helper now provides --no-xxx so that git-completion.bash
can drop the extra custom --no-xxx in the script. It adds a lot more
--no-xxx than what's current provided by the git-completion.bash
script. We'll trim that down later.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>