bug#6204: vc-dir always splits the frame

[martin rudalics <rudalics@gmx.at>]

 > Rather than let-binding some Lisp-manipulated "config" var, I was
 > thinking of passing special parameters to display-buffer (I'd rather
 > avoid dynamic scoping whenever possible).

An additional argument is always fine with me.  So this part is solved.

 > You mean how could the calling code till display-buffer whether to
 > "prefer <foo> unless overridden by the user's config" or to "obey <foo>
 > regardless of the user's config"?

Precisely.

 > I guess the special parameters passed to display-buffer would have to be
 > different for the two different cases.

Fine with me too.

If the overriding case and the user option coincide in principle but the
user option has additional values like in the foo-other-frame case with
the user specifying certain frame parameters what shall we do?  Apply
the frame parameters?  Should the additional argument also allow such
frame parameters?  Should, as a rule, the potential argument's values be
identic to the values allowed by the option?

Remains the question how to make customizing monsters like
`special-display-regexps' simpler.  Currently we're heading in the
opposite direction :-(

martin