patch 8.1.1915: 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 027cdc8..bbdbfb8 100644
--- a/src/testdir/test_diffmode.vim
+++ b/src/testdir/test_diffmode.vim
@@ -674,7 +674,7 @@
   call diff_hlID(1, 2)->synIDattr("name")->assert_equal("DiffText")
   call diff_hlID(2, 1)->synIDattr("name")->assert_equal("")
   call diff_hlID(3, 1)->synIDattr("name")->assert_equal("DiffAdd")
-  call diff_hlID(4, 1)->synIDattr("name")->assert_equal("")
+  eval 4->diff_hlID(1)->synIDattr("name")->assert_equal("")
 
   wincmd w
   call assert_equal(synIDattr(diff_hlID(1, 1), "name"), "DiffChange")
@@ -693,7 +693,7 @@
   diffthis
   redraw
 
-  call assert_equal([0, 0, 0, 0, 0, 0, 0, 1, 0], map(range(-1, 7), 'diff_filler(v:val)'))
+  call assert_equal([0, 0, 0, 0, 0, 0, 0, 1, 0], map(range(-1, 7), 'v:val->diff_filler()'))
   wincmd w
   call assert_equal([0, 0, 0, 0, 2, 0, 0, 0], map(range(-1, 6), 'diff_filler(v:val)'))