Since we can't automatically extract diff options for completion yet,
difftool will take all options from $__git_diff_common_options. This
brings _a lot_ more completable options to difftool.
--ignore-submodules is added to $__git_diff_common_options to avoid
regression in difftool. But it's a good thing anyway even for other
diff commands.
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>