patch 8.1.0256: using setline() in TextChangedI splits undo

Problem:    Using setline() in TextChangedI splits undo.
Solution:   Use another solution for undo not working properly.
diff --git a/src/testdir/test_autocmd.vim b/src/testdir/test_autocmd.vim
index b52493f..3d650e4 100644
--- a/src/testdir/test_autocmd.vim
+++ b/src/testdir/test_autocmd.vim
@@ -1329,10 +1329,8 @@
   call assert_equal('(', getline(1))
   call assert_equal('x)', getline(2))
   undo
-  call assert_equal('(', getline(1))
-  call assert_equal('', getline(2))
-  undo
   call assert_equal('', getline(1))
+  call assert_equal('', getline(2))
 
   call test_override('starting', 0)
   bwipe!