bug#1114: M-` results in huge minibuffer

[Geoff Gole]

>
> I think the reason might be the call to fit-window-to-buffer in
> Electric-pop-up-window.
>

I took a look. The minibuffer indeed changes size due to a call to
enlarge-window inside fit-window-to-buffer.

It seems that while enlarge-window doesn't usually touch the
minibuffer, it will take effect when inside a completing-read. For
example:

  (let ((win (selected-window)))
    (completing-read "Test prompt : "
     (lambda (&rest *)
       (with-selected-window win
         (enlarge-window -10))))) ; minibuffer will grow ten lines

Further up the call stack I see tmm-prompt makes such a call to
completing-read, so it seems likely that is the problem. I have no
fix, though.