patch 8.2.4941: '[ and '] marks may be wrong after undo

Problem:    '[ and '] marks may be wrong after undo.
Solution:   Adjust the '[ and '] marks if needed. (closes #10407, closes #1281)
diff --git a/src/testdir/test_undo.vim b/src/testdir/test_undo.vim
index bfcc538..596944c 100644
--- a/src/testdir/test_undo.vim
+++ b/src/testdir/test_undo.vim
@@ -756,4 +756,21 @@
   bwipe!
 endfunc
 
+func Test_undo_mark()
+  new
+  " The undo is applied to the only line.
+  call setline(1, 'hello')
+  call feedkeys("ggyiw$p", 'xt')
+  undo
+  call assert_equal([0, 1, 1, 0], getpos("'["))
+  call assert_equal([0, 1, 1, 0], getpos("']"))
+  " The undo removes the last line.
+  call feedkeys("Goaaaa\<Esc>", 'xt')
+  call feedkeys("obbbb\<Esc>", 'xt')
+  undo
+  call assert_equal([0, 2, 1, 0], getpos("'["))
+  call assert_equal([0, 2, 1, 0], getpos("']"))
+  bwipe!
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab