diff3: Use a format string to quiet a compiler warning

And bump WARNS to 2

contrib/diff/src/diff3.c

@@ -1405,7 +1405,7 @@ output_diff3 (FILE *outputfile, struct diff3_block *diff,
 	      line = 0;
 	      do
 		{
-		  fprintf (outputfile, line_prefix);
+		  fprintf (outputfile, "%s", line_prefix);
 		  cp = D_RELNUM (ptr, realfile, line);
 		  length = D_RELLEN (ptr, realfile, line);
 		  fwrite (cp, sizeof (char), length, outputfile);

gnu/usr.bin/diff3/Makefile

@@ -17,6 +17,6 @@ CFLAGS+=-I${SRCTOP}/contrib/diff/lib
 CFLAGS+=-DHAVE_CONFIG_H
 CFLAGS+=-DDEFAULT_DIFF_PROGRAM=\"/usr/bin/diff\"
 
-WARNS?=	0
+WARNS?=	2
 
 .include <bsd.prog.mk>