custom-save-variables: pretty-printing long values

[Adam Spiers <emacs-devel@adamspiers.org>]

This simple tweak to cus-edit ensures that custom variable values
which are over 60 bytes long get written to `custom-file' using
`indent-pp-sexp'.  For example:

    (custom-set-variables
     ;; custom-set-variables was added by Custom.
     ;; If you edit it by hand, you could mess it up, so be careful.
     ;; Your init file should contain only one such instance.
     ;; If there is more than one, they won't work right.
[snipped]
     '(c-default-style
       (quote
        ((c-mode . "linux")
         (java-mode . "java")
         (awk-mode . "awk")
         (other . "gnu"))))

This not only makes the file much easier to read, but is particularly
useful when the file is tracked by a version control system, since
then the diffs become much more manageable.  For example, my setting
for `org-agenda-custom-commands' is 349 lines long (14989 characters).
When I recustomise that variable, the output of `git diff' is
almost completely illegible.

Are there any problems with this approach?  

Thanks!
Adam


diff --git a/lisp/cus-edit.el b/lisp/cus-edit.el
index 4ed72be..673ea44 100644
--- a/lisp/cus-edit.el
+++ b/lisp/cus-edit.el
@@ -4590,7 +4590,15 @@ This function does not save the buffer."
 	    (princ " '(")
 	    (prin1 symbol)
 	    (princ " ")
-	    (prin1 (car value))
+	    (let ((val (prin1-to-string (car value))))
+	      (if (< (length val) 60)
+		  (insert val)
+		(newline-and-indent)
+		(let ((beginning-of-val (point)))
+		  (insert val)
+		  (save-excursion
+		    (goto-char beginning-of-val)
+		    (indent-pp-sexp 1)))))
 	    (when (or now requests comment)
 	      (princ " ")
 	      (prin1 now)