bug#38193: 26.2; Editing a message in RMAIL should trigger an update to its summary line

[Ed Sabol via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Steps:
1. Open an existing RMAIL file with multiple messages in rmail-mode.
2. Press "h" to generate the corresponding rmail-summary buffer.
3. Hit "e" to edit some message.
4. Change the Subject: header of the message in some noticeable way.
5. Press C-c C-c to finish editing the message.

Result:
The corresponding summary line for the edited message is not updated to
reflect the message's new Subject: header. I expect that it should.
It used to work like that in older Emacs, pre 22.x for sure, so I
consider this to be a regression, albeit a very old one!

Same for other headers like From: and To:.


In GNU Emacs 26.2 (build 2, x86_64-pc-linux-gnu, X toolkit)
 of 2019-04-12 built on heasarcdev
Windowing system distributor 'Scientific Linux', version 11.0.11704000
System Description:	Scientific Linux release 6.10 (Carbon)

Recent messages:
Parsing /Home/lhea2/sabol/.mailrc... done
Mark set
Editing: Type C-c C-c to return to Rmail, C-c C-] to abort
Computing summary lines...done
Editing: Type C-c C-c to return to Rmail, C-c C-] to abort
Computing summary lines...done
Saving file /Home/lhea2/sabol/Mail/RMAIL...
Wrote /Home/lhea2/sabol/Mail/RMAIL [2 times]
Computing summary lines...done
Making completion list...

Configured using:
 'configure --prefix=/software/usr/contrib
 --exec-prefix=/software/usr/contrib/linux-6.x --with-x-toolkit=athena
 --with-toolkit-scroll-bars=no --with-jpeg=no --with-gif=no
 --with-tiff=no --with-gnutls=no'

Configured features:
XAW3D XPM PNG SOUND GSETTINGS GLIB NOTIFY LIBSELINUX LIBXML2 FREETYPE
XFT ZLIB LUCID X11 XDBE XIM THREADS

Important settings:
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: @im=none
  locale-coding-system: utf-8-unix

Major mode: RMAIL Summary

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  buffer-read-only: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort emacsbug sendmail rmailedit mail-extr qp rmailsum rmailmm
message rmc puny seq byte-opt gv bytecomp byte-compile cconv cl-loaddefs
cl-lib dired dired-loaddefs format-spec rfc822 mml easymenu mml-sec
password-cache epa derived epg epg-config gnus-util mm-decode mm-bodies
mm-encode mailabbrev gmm-utils mailheader mail-parse rfc2231 rmail
rmail-loaddefs rfc2047 rfc2045 ietf-drums mm-util mail-prsvr mail-utils
elec-pair time-date mule-util tooltip eldoc electric uniquify ediff-hook
vc-hooks lisp-float-type mwheel term/x-win x-win term/common-win x-dnd
tool-bar dnd fontset image regexp-opt fringe tabulated-list replace
newcomment text-mode elisp-mode lisp-mode prog-mode register page
menu-bar rfn-eshadow isearch timer select scroll-bar mouse jit-lock
font-lock syntax facemenu font-core term/tty-colors frame cl-generic
cham georgian utf-8-lang misc-lang vietnamese tibetan thai tai-viet lao
korean japanese eucjp-ms cp51932 hebrew greek romanian slovak czech
european ethiopic indian cyrillic chinese composite charscript charprop
case-table epa-hook jka-cmpr-hook help simple abbrev obarray minibuffer
cl-preloaded nadvice loaddefs button faces cus-face macroexp files
text-properties overlay sha1 md5 base64 format env code-pages mule
custom widget hashtable-print-readable backquote threads inotify
dynamic-setting font-render-setting x-toolkit x multi-tty
make-network-process emacs)

Memory information:
((conses 16 117388 19492)
 (symbols 48 20833 2)
 (miscs 40 5507 5869)
 (strings 32 36097 7923)
 (string-bytes 1 1291947)
 (vectors 16 15244)
 (vector-slots 8 529193 12588)
 (floats 8 58 240)
 (intervals 56 5821 11)
 (buffers 992 16)
 (heap 1024 31454 10375))