git.c: add hidden option --list-parseopt-builtins

git.c: add hidden option --list-parseopt-builtins

This is another step to help automate git-completion.bash. This option
gives a list of all builtin commands that do use parse_options(),
which supports another hidden option --git-completion-helper. The
output is prepared for easy consumption by git-completion.bash and
separates items by spaces instead of \n

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