patch 8.0.0205: wrong behavior after :undojoin

Problem:    After :undojoin some commands don't work properly, such as :redo.
            (Matthew Malcomson)
Solution:   Don't set curbuf->b_u_curhead. (closes #1390)
diff --git a/src/testdir/test_undo.vim b/src/testdir/test_undo.vim
index f2ac6a8..06732af 100644
--- a/src/testdir/test_undo.vim
+++ b/src/testdir/test_undo.vim
@@ -176,7 +176,17 @@
   call assert_equal(['aaaa', 'bbbb', 'cccc'], getline(2, '$'))
   call feedkeys("u", 'xt')
   call assert_equal(['aaaa'], getline(2, '$'))
-  close!
+  bwipe!
+endfunc
+
+func Test_undojoin_redo()
+  new
+  call setline(1, ['first line', 'second line'])
+  call feedkeys("ixx\<Esc>", 'xt')
+  call feedkeys(":undojoin | redo\<CR>", 'xt')
+  call assert_equal('xxfirst line', getline(1))
+  call assert_equal('second line', getline(2))
+  bwipe!
 endfunc
 
 func Test_undo_write()