bug#6204: vc-dir always splits the frame

[martin rudalics <rudalics@gmx.at>]

 >>> 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.
 >
 > Indeed.  A good way would be to provide a way to override pop-up-windows
 > (both nil and t) in special-display-regexps.  We already have
 > (same-window . t), so we only need to add the other half.

In Juri's `switch-to-buffer-other-window' case such options would be
useless anyway.

 >> 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.
 >
 > But maybe the user would like to follow the application's special
 > handling, even tho she modified her pop-up-windows for other reasons.
 > I think it's much better to use 2 separate variables, where the
 > user-controlled var is never modified by Elisp code.

Applications that do care enough should either (1) use `split-window'
and `set-window-buffer' or (2) provide a separate customizable option
like `foo-pop-up-windows' and let the user decide whether this should
pop up a new window and where.

martin