patch 8.1.1988: :startinsert! does not work the same way as "A"

Problem:    :startinsert! does not work the same way as "A".
Solution:   Use the same code to move the cursor. (closes #4896)
diff --git a/src/testdir/test_edit.vim b/src/testdir/test_edit.vim
index 799642d..6ee5848 100644
--- a/src/testdir/test_edit.vim
+++ b/src/testdir/test_edit.vim
@@ -1480,3 +1480,18 @@
 
   close!
 endfunc
+
+func Test_edit_startinsert()
+  new
+  set backspace+=start
+  call setline(1, 'foobar')
+  call feedkeys("A\<C-U>\<Esc>", 'xt')
+  call assert_equal('', getline(1))
+
+  call setline(1, 'foobar')
+  call feedkeys(":startinsert!\<CR>\<C-U>\<Esc>", 'xt')
+  call assert_equal('', getline(1))
+
+  set backspace&
+  bwipe!
+endfunc