patch 8.1.0570: 'commentstring' not used when adding fold marker

Problem:    'commentstring' not used when adding fold marker. (Maxim Kim)
Solution:   Only use empty 'comments' middle when leader is empty. (Christian
            Brabandt, closes #3670)
diff --git a/src/testdir/test_fold.vim b/src/testdir/test_fold.vim
index 0384b4f..f8664d4 100644
--- a/src/testdir/test_fold.vim
+++ b/src/testdir/test_fold.vim
@@ -695,3 +695,20 @@
   call StopVimInTerminal(buf)
   call delete('Xtest_folds_with_rnu')
 endfunc
+
+func Test_folds_marker_in_comment2()
+  new
+  call setline(1, ['Lorem ipsum dolor sit', 'Lorem ipsum dolor sit', 'Lorem ipsum dolor sit'])
+  setl fen fdm=marker
+  setl commentstring=<!--%s-->
+  setl comments=s:<!--,m:\ \ \ \ ,e:-->
+  norm! zf2j
+  setl nofen
+  :1y
+  call assert_equal(['Lorem ipsum dolor sit<!--{{{-->'], getreg(0,1,1))
+  :+2y
+  call assert_equal(['Lorem ipsum dolor sit<!--}}}-->'], getreg(0,1,1))
+
+  set foldmethod&
+  bwipe!
+endfunc