bug#6204: vc-dir always splits the frame

[martin rudalics]

> Now after let-binding `pop-up-windows' to nil around the `pop-to-buffer'
> call, what if someone might want to restore the old behavior.

I currently use the following semantics for `pop-up-windows':

---------------------------------------------------------------
Non-nil means `display-buffer' is allowed to make a new window.
A non-empty list specifies the windows `display-buffer' will
consider for splitting.  The following entries are supported
where "frame" refers to the frame chosen to display the buffer:

 largest ...... largest window
 lru .......... least recently used window
 selected ..... frame's selected window
 root ......... frame's root window

The default value t stands for the list `(largest lru)'.  This
means that `display-buffer' will first try to split the largest
window and, if that fails, the least recently used window.
---------------------------------------------------------------

The intended use is that an application calling `display-buffer'
or `pop-to-buffer' could bind this as

(let  ((pop-up-windows
        (if (eq pop-up-windows t)
            application-dependent-value
          pop-up-windows)))

that is, the default value can be overridden by the application,
user customizations are respected.

And obviously Lisp code should not call `switch-to-buffer' and
its colleagues.

martin