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

[Eli Zaretskii]

> From: Fritz Stelzer <brotzeitmacher@gmail.com>
> Date: Wed, 8 Nov 2017 10:43:29 +0100
> 
> master c89870d7ef8b59febba9060f0da25b24403c2e08
> Author:     brotzeit <brotzeitmacher@gmail.com>
> AuthorDate: Wed Nov 8 10:39:57 2017 +0100
> Commit:     brotzeit <brotzeitmacher@gmail.com>
> CommitDate: Wed Nov 8 10:39:57 2017 +0100
> 
> Parent:     83dc3f0e9b * lisp/net/tramp-cmds.el, lisp/allout.el: Avoid 
> custom-set-variables
> Merged:     master
> Containing: master
> Follows:    emacs-25.1 (130661)
> 
> Make xref-show-xrefs-function a defcustom
> 
> 1 file changed, 3 insertions(+), 2 deletions(-)
> lisp/progmodes/xref.el | 5 +++--
> 
> modified   lisp/progmodes/xref.el
> @@ -738,8 +738,9 @@ xref--show-xref-buffer
>  ;; The controversial multiple definitions case is handed off to
>  ;; xref-show-xrefs-function.
>  
> -(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.)