parse-options: add --git-completion-helper-all

parse-options: add --git-completion-helper-all

--git-completion-helper excludes hidden options, such as --allow-empty
for git commit. This is typically helpful, but occasionally we want
auto-completion for obscure flags. --git-completion-helper-all returns
all options, even if they are marked as hidden or nocomplete.

Signed-off-by: Ryan Zoeller <rtzoeller@rtzoeller.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>