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>