diff: honour flags with -q

Previously -q (just print a line when files differ) ignored flags like
-w (ignore whitespace). Avoid the D_BRIEF short-circuit when flags are
in effect.

PR: 252515
Reported by: Scott Aitken
Reviewed by: kevans
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D28064

(cherry picked from commit 12a8d3027d414abd15798de1b6d8b01151bdac50)