diff: remove DIFF_OPT_TST macro

diff: remove DIFF_OPT_TST macro

Remove the `DIFF_OPT_TST` macro and instead access the flags directly.
This conversion is done using the following semantic patch:

	@@
	expression E;
	identifier fld;
	@@
	- DIFF_OPT_TST(&E, fld)
	+ E.flags.fld

	@@
	type T;
	T *ptr;
	identifier fld;
	@@
	- DIFF_OPT_TST(ptr, fld)
	+ ptr->flags.fld

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>