bug#29206: Make xref-show-xrefs-function a defcustom

[Eli Zaretskii <eliz@gnu.org>]

> Cc: 29206@debbugs.gnu.org
> From: Dmitry Gutov <dgutov@yandex.ru>
> Date: Sun, 19 Nov 2017 01:42:17 +0200
> 
> On 11/18/17 12:14 PM, Eli Zaretskii wrote:
> 
> >> -(defvar xref-show-xrefs-function 'xref--show-xref-buffer
> >> -  "Function to display a list of xrefs.")
> >> +(defcustom xref-show-xrefs-function 'xref--show-xref-buffer
> >> +  "Function to display a list of xrefs."
> >> +  :type 'function)
> >>   
> >>   (defvar xref--read-identifier-history nil)
> > 
> > Dmitry, any comments on this?  (If accepted, the defcustom should have
> > a :version tag.)
> 
> I don't really see the point, yet: we don't offer any alternative 
> functions, and if the user is going to write one, `setq' is just as easy 
> to do.
> 
> Further, applying this kind of minor change (and finding it important) 
> seems like it would indicate that xref is kind of "stable", which it is 
> not, so far.

Fine with me, then please close as "wontfix".