completion: fix __gitcomp_builtin no longer consider extra options

completion: fix __gitcomp_builtin no longer consider extra options

__gitcomp_builtin() has the main completion list provided by

    git xxx --git-completion-helper

but the caller can also add extra options that is not provided by
--git-completion-helper. The only call site that does this is "git
difftool" completion.

This support is broken by b221b5ab9b (completion: collapse extra
--no-.. options - 2018-06-06), which adds a special value "--" to mark
that the rest of the options can be hidden by default. The commit
forgets the fact that extra options are appended after
"$(git xxx --git-completion-helper)", i.e. after this "--", and will
be incorrectly hidden as well.

Prepend the extra options before "$(git xxx --git-completion-helper)"
to avoid this.

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