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
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D28064