vc-annotate-background customization is incorrect

[Gustav Hållberg]

In ftp://ftp.gnu.org/pub/gnu/emacs/emacs-22.1.tar.gz the (defcustom) for 
vc-annotate-background incorrectly only allows string values.

A better choice would be to allow nil (for using the default background) 
or the "color" type.

I attach a suggested patch.

- Gustav

--- emacs-22.1/lisp/vc.el~      2007-05-14 16:56:29.000000000 +0200
+++ emacs-22.1/lisp/vc.el       2008-03-17 16:01:54.000000000 +0100
@@ -689,7 +689,7 @@
 (defcustom vc-annotate-background "black"
   "Background color for \\[vc-annotate].
 Default color is used if nil."
-  :type 'string
+  :type '(choice (const :tag "Default background" nil) (color))
   :group 'vc)
 
 (defcustom vc-annotate-menu-elements '(2 0.5 0.1 0.01)