patch 8.1.1925: 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_changelist.vim b/src/testdir/test_changelist.vim
index dd6ea96..ce77c1f 100644
--- a/src/testdir/test_changelist.vim
+++ b/src/testdir/test_changelist.vim
@@ -8,8 +8,8 @@
 
   bwipe!
   enew
-  call assert_equal([], getchangelist(10))
-  call assert_equal([[], 0], getchangelist('%'))
+  call assert_equal([], 10->getchangelist())
+  call assert_equal([[], 0], getchangelist())
 
   call writefile(['line1', 'line2', 'line3'], 'Xfile1.txt')
   call writefile(['line1', 'line2', 'line3'], 'Xfile2.txt')