patch 8.1.1993: more functions can be used as methods

Problem:    More functions can be used as methods.
Solution:   Make various functions usable as a method.
diff --git a/src/testdir/test_diffmode.vim b/src/testdir/test_diffmode.vim
index bbdbfb8..8280a2c 100644
--- a/src/testdir/test_diffmode.vim
+++ b/src/testdir/test_diffmode.vim
@@ -330,7 +330,7 @@
   call setline(1, ['One', '', 'Two', 'Three'])
   diffthis
   redraw
-  call assert_notequal(normattr, screenattr(1, 1))
+  call assert_notequal(normattr, 1->screenattr(1))
   diffoff!
   redraw
   call assert_equal(normattr, screenattr(1, 1))