Update mundo undo tree when main window is changed.

[dsummersl]

Ideal behavior:

1. Start vim with no file. Make a few changes.
2. call :MundoShow. Changes should be seen.
3. open a new file :e new.txt.
4. The mundo window still reflects the undo state of the empty buffer - ideally it would show the new/empty undo history of new.txt.