Actually clean difftool on make clean

Makefile.in

@@ -82,7 +82,7 @@ utf8-iconv:
 
 clean:
 	test ! -d build || rm -r build/
-	for d in @subdirs@; do $(MAKE) -C $$d clean; done
+	for d in @subdirs@ testsuite/difftool; do $(MAKE) -C $$d clean; done
 
 LIBGITDIRS = `ls -f libraries/git`