bug#42016: 26.3; In latex-mode, fill-paragraph changes code to comments

[Nate Eldredge]

In latex-mode, using fill-paragraph (M-q) on a non-comment line
following a comment will sometimes merge the non-comment line into the
comment, thus messing up the code.

To reproduce, try opening the following file (with C-x f):

----- snip -----
%% Demonstrate fill bug with GNU emacs
%% Open this file in latex-mode.  Position point on the "yyy" line and hit M-q.
\documentclass{article}
\begin{document}
\begin{environment}
  %% xxx
  yyy
\end{environment}
\end{document}
----- snip ------

Select latex-mode if necessary, position point on the 'yyy' line, and
hit M-q.  The result is

\begin{environment}
  %% xxx yyy
\end{environment}

so that the "live" code 'yyy' has unintentionally been commented out.
This is bad, as M-q should not change the semantics of the file.


In GNU Emacs 26.3 (build 2, x86_64-pc-linux-gnu)
 of 2020-03-26, modified by Debian built on lcy01-amd64-020
System Description:     Ubuntu 20.04 LTS

Recent messages:
Loading /etc/emacs/site-start.d/00debian.el (source)...done
Loading /etc/emacs/site-start.d/50autoconf.el (source)...done
For information about GNU Emacs and the GNU system, type C-h C-a.
Undo!

Configured using:
 'configure --build x86_64-linux-gnu --prefix=/usr
 --sharedstatedir=/var/lib --libexecdir=/usr/lib
 --localstatedir=/var/lib --infodir=/usr/share/info
 --mandir=/usr/share/man --enable-libsystemd --with-pop=yes
 
--enable-locallisppath=/etc/emacs:/usr/local/share/emacs/26.3/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/26.3/site-lisp:/usr/share/emacs/site-lisp
 --with-sound=alsa --without-gconf --with-mailutils --build
 x86_64-linux-gnu --prefix=/usr --sharedstatedir=/var/lib
 --libexecdir=/usr/lib --localstatedir=/var/lib
 --infodir=/usr/share/info --mandir=/usr/share/man --enable-libsystemd
 --with-pop=yes
 
--enable-locallisppath=/etc/emacs:/usr/local/share/emacs/26.3/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/26.3/site-lisp:/usr/share/emacs/site-lisp
 --with-sound=alsa --without-gconf --with-mailutils --with-x=no
 --without-gsettings 'CFLAGS=-g -O2
 -fdebug-prefix-map=/build/emacs-mEZBk7/emacs-26.3+1=. -fstack-protector-strong
 -Wformat -Werror=format-security -Wall' 'CPPFLAGS=-Wdate-time
 -D_FORTIFY_SOURCE=2' 'LDFLAGS=-Wl,-Bsymbolic-functions -Wl,-z,relro''

Configured features:
JPEG SOUND GPM DBUS NOTIFY ACL LIBSELINUX GNUTLS LIBXML2 ZLIB XIM
THREADS LIBSYSTEMD LCMS2

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

Major mode: LaTeX

Minor modes in effect:
  shell-dirtrack-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
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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 rmail rmail-loaddefs mm-decode mm-bodies mm-encode
mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail rfc2047
rfc2045 ietf-drums mm-util mail-prsvr mail-utils tex-mode compile
tool-bar shell pcomplete comint regexp-opt ansi-color ring latexenc
term/xterm xterm time-date elec-pair mule-util tooltip eldoc electric
uniquify ediff-hook vc-hooks lisp-float-type tabulated-list replace
newcomment text-mode elisp-mode lisp-mode prog-mode register page
menu-bar rfn-eshadow isearch timer select 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 dbusbind
inotify lcms2 multi-tty make-network-process emacs)

Memory information:
((conses 16 105934 5924)
 (symbols 48 20352 1)
 (miscs 40 43 157)
 (strings 32 31264 945)
 (string-bytes 1 849390)
 (vectors 16 13201)
 (vector-slots 8 461982 4418)
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