bug#65071: 29.1; RMAIL changed on disk leads to rmail-error-bad-format

[emacs-bug@rulnick.com]

I hope the receipe below is clearer. In Emacs 29.1, starting with no
~/RMAIL file and empty primary mail spool file (please note that '$'
indicates Bash prompt in Terminal independent of Emacs):

  $ mail me@foo ... # send a message to myself

  $ emacs -Q

  (in Emacs) M-x rmail RET # retrieves/reads message

  $ cat /dev/null > ~/RMAIL # clear RMAIL file

  $ mail me@foo ... # send another message to myself

  (in Emacs) g

    "File RMAIL changed on disk. Reread from disk? (yes or no) " yes RET

    "rmail-error-bad-format: Message is not a valid RFC 822 (or later)
    message"; Emacs mode changes from RMAIL to Fundamental

At this point the ~/RMAIL file on disk looks good (proper RFC 822 file),
but the ~/RMAIL file as visited in buffer looks bad, as can be seen
(e.g., upon immediate save) to be missing top "From " and other header
lines ("Return-Path:", "Received:", etc.). I have many years experience
with Emacs and RMAIL and this behavior is new as of 29.1 or another very
recent version of Emacs.