diff: set cflag when passed to diff

In b5541f456d when flags were converted to
be boolean, the setting of cflag as an integer was removed, but no
boolean set was added.

This effects the output format of dates, but the context type was set to
D_CONTEXT so other functionality worked.

Reviewed by:	bapt
Sponsored by:	Klara Inc.
Differential Revision:	https://reviews.freebsd.org/D34481

usr.bin/diff/diff.c

@@ -159,6 +159,7 @@ main(int argc, char **argv)
 		case 'c':
 			if (FORMAT_MISMATCHED(D_CONTEXT))
 				conflicting_format();
+			cflag = true;
 			diff_format = D_CONTEXT;
 			if (optarg != NULL) {
 				l = strtol(optarg, &ep, 10);