grep: fix handling of -b flag

Output buffering is automatically disabled when reading from stdin.
In this case, supplying the -b flag ought to be redundant.  However,
since Bflag was being XORed into the flag set - rather than simply
ORed - supplying -b would actually enable output buffering.

sys/src/cmd/grep/main.c

@@ -91,7 +91,7 @@ search(char *file, int flag)
 	}
 
 	if(flags['b'])
-		flag ^= Bflag;		/* dont buffer output */
+		flag |= Bflag;		/* dont buffer output */
 	if(flags['c'])
 		flag |= Cflag;		/* count */
 	if(flags['h'])