Do not generate empty diffs.

Index: files/mkpatches.pl
===================================================================
RCS file: /cvsroot/pkgsrc/pkgtools/pkgdiff/files/mkpatches.pl,v
retrieving revision 1.21
diff -u -p -r1.21 mkpatches.pl
--- files/mkpatches.pl	10 Feb 2016 16:00:10 -0000	1.21
+++ files/mkpatches.pl	10 Feb 2016 22:09:04 -0000
@@ -266,9 +266,7 @@ sub make_patch # new old patchfile diff
 
     if ("$diff" eq "") {
 	print "$old and $new don't differ\n";
-	if (-f "$patchdir/$patchfile.orig") {
-	    rename "$patchdir/$patchfile.orig", "$patchdir/$patchfile";
-	}
+	return;
     }
     if (not -f "$patchdir/$patchfile.orig") {
 	system("touch", "$patchdir/$patchfile.orig");