bug#2941: rmail's new handling of Babyl files is odd

[Glenn Morris]

Richard M Stallman wrote:

>     C-u M-x rmail RET foo RET  ; foo is a Babyl file with > 1 message
>     d
>     s
>     y
>
> Does this patch give good results?

It fixes the above sequence; however I can still go from Babyl to mbox
with no prompting by doing:

C-u M-x rmail RET foo RET
d
x
y
C-x C-s


> *** rmail.el.~1.534.~ 2009-04-10 10:13:07.000000000 -0400
> --- rmail.el  2009-04-14 16:15:38.000000000 -0400
> ***************
> *** 1458,1463 ****
> --- 1458,1464 ----
>     "Expunge and save RMAIL file."
>     (interactive)
>     (set-buffer rmail-buffer)
> +   (rmail-modify-format)
>     (rmail-expunge)
>     ;; No need to swap buffers: rmail-write-region-annotate takes care of it.
>     ;; (rmail-swap-buffers-maybe)