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

[<emacs-bug@rulnick.com>]

In Emacs 29.1, starting with no ~/RMAIL file and empty primary mail
spool file ('$' indicates command line terminal prompt outside Emacs):

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

  emacs -Q

  M-x RMAIL # retrieves/reads message

  $ cat /dev/null > ~/RMAIL # clear (or otherwise modify) RMAIL file

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

  M-x RMAIL

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

    "rmail-error-bad-format: Message is not a valid RFC 822 (or later)
    message" and:

      mode changes from RMAIL to fundamental

      ~/RMAIL file on disk looks good (proper RFC 822 file)

      ~/RMAIL file as visited in buffer looks bad, can be seen (e.g.,
        upon immediate save) to be missing top "From " and other header
        lines ("Return-Path:", "Received:", etc.)

In GNU Emacs 29.1 (build 1, aarch64-apple-darwin21.6.0, NS
 appkit-2113.60 Version 12.6.6 (Build 21G646)) of 2023-07-30 built on
 armbob.lan
Windowing system distributor 'Apple', version 10.3.2113
System Description:  macOS 12.6.8

Configured using:
 'configure --with-ns '--enable-locallisppath=/Library/Application
 Support/Emacs/${version}/site-lisp:/Library/Application
 Support/Emacs/site-lisp' --with-modules 'CFLAGS=-DFD_SETSIZE=10000
 -DDARWIN_UNLIMITED_SELECT' --with-x-toolkit=no'

Configured features:
ACL GLIB GMP GNUTLS JPEG JSON LIBXML2 MODULES NOTIFY KQUEUE NS PDUMPER
PNG RSVG THREADS TIFF TOOLKIT_SCROLL_BARS TREE_SITTER ZLIB

Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix