bug#445: Invalid read syntax in savehist-mode's history file

[Chong Yidong]

> I found this line in my ~/.emacs.d/history. I've trimmed the list to
> include only the problematic element and formatted it for email.
>
> (setq command-history
>       ...  
>       '(describe-key-briefly
>         '[mode-line (down-mouse-1
>                      (#<window 199> mode-line (110 . 617)
>                                -97640112
>                                ("Calc: 12 Deg               " . 6)
>                                41 (13 . 47) nil (5 . 9) (8 . 16)))] nil 1)
>       ...)
>
> The lisp reader complains about the #<window 199>.

AFAIK, savehist-mode doesn't save command-history.  Please provide a
recipe for reproducing this bug.