patch 9.1.0672: marker folds may get corrupted on undo

Problem:  marker folds may get corrupted on undo (Yousef Mohammed)
Solution: when adjusting folds, make sure that line1 is the lower limit
          and line2 is the upper line limit. In particular, line2 should
          not be able to get smaller than line1.

fixes: #15455
closes: #15466

Signed-off-by: Christian Brabandt <cb@256bit.org>

src/testdir/test_fold.vim

@@ -1928,4 +1928,25 @@ func Test_cursor_down_fold_eob()
   call assert_equal(4, line('.'))
   bwipe!
 endfunc
+
+" issue: #15455
+func Test_cursor_fold_marker_undo()
+  new
+  call setline(1, ['{{{', '', 'This is a Line', '', 'This is a Line', '', '}}}'])
+  let &ul=&ul
+  setl foldmethod=marker
+  call cursor(2, 1)
+  norm! zo1vjdu
+  call assert_equal(1, foldlevel('.'))
+  bwipe!
+  new
+  call setline(1, ['', '{{{', '', 'This is a Line', '', 'This is a Line', '', '}}}'])
+  let &ul=&ul
+  setl foldmethod=marker
+  call cursor(3, 1)
+  norm! zo
+  norm! vjdu
+  call assert_equal(1, foldlevel('.'))
+  bwipe!
+endfunc