diff --no-index: use parse_options() instead of diff_opt_parse()

While at there, move exit() back to the caller. It's easier to see the
flow that way than burying it in diff-no-index.c

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