parse-options: support --git-completion-helper
This option is designed to be used by git-completion.bash. For many
simple cases, what we do in there is usually
__gitcomp "lots of completion options"
which has to be manually updated when a new user-visible option is
added. With support from parse-options, we can write
__gitcomp "$(git command --git-completion-helper)"
and get that list directly from the parser for free. Dangerous/Unpopular
options could be hidden with the new "NOCOMPLETE" flag.
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>