diff.h: avoid bit fields in struct diff_flags

diff.h: avoid bit fields in struct diff_flags

Bitfield addresses cannot be passed around in a pointer. This makes it
hard to use parse-options to set/unset them. Turn this struct to
normal integers. This of course increases the size of this struct
multiple times, but since we only have a handful of diff_options
variables around, memory consumption is not at all a concern.

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