UNDO CAN STOP WORKING

[Zoltan Koszegi]

This can be easily recreated on the CK demo page with IE9. STR:

1. On the demo page in IE9, switch to the full featured editor.
2. Select all and delete to make the editor empty.
3. Copy some text from Notepad and paste it 3 times.
4. Click undo. Notice the last pasted instance disappears but the cursor doesn't jump back to the end of the text.
5. Further clicks on undo have no effect.

I attached undo_test_note.txt. I can reproduce with this junk of text.

[Jakub Ś]

Dup of #10301.