bug#16909: 24.3; scrolling *Completions* window with tab sometimes chooses a wrong window

[martin rudalics]

> I just posted this setq line as a workaround. And why does it break? Because 
if you set it then C-M-v will always try to scroll *Completions* and not regular 
other-window buffer which is useful sometimes (e.g. while using help, an approach 
mentioned even in tutorial).

So it seems there are two ways to tackle this problem:

(1) Kill the *Completions* buffer when we're done so it won't be
    inadvertently displayed by `other-window-for-scrolling'.

(2) Bind `other-window-scroll-buffer' temporarily only as long as the
    *Completions* buffer is shown.

martin