diff: use add_diff_options() in diff_opt_parse()
Prepare the removal of the parseopts member of struct diff_options by
using the API function add_diff_options() instead of accessing it
directly to get the command line option definitions. Building the copy
by concatenating with an empty option array is slightly awkward, but
simpler than a non-concat version of add_diff_options() would be to use
in places that need concatenation.
Suggested-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>