Redo fixed with a strategy similar (but not equal) to the one for undo:

Redo fixed with a strategy similar (but not equal) to the one for undo:
1. we save the position of all the marks
2. we redo
3. we undo
4. we save the position of the cursor after the undo
5. if necessary, we restore the initial position of all the marks,
   we move the cursor to its position after the undo and then we goto
6. we redo