bug#6204: vc-dir always splits the frame

[Stefan Monnier]

> `switch-to-buffer' is allowed to reuse a weakly dedicated window while
> `display-buffer' isn't.

Indeed, and that's on purpose.  It's basically that switch-to-buffer has
a strong `same-window' parameter.  I don't think display-buffer and
special-display-regexps have any way to specify such a request, so we'll
need to add it.

> This gets complicated :-(

It needs to be cleaned up, yes.


        Stefan