bug#23526: 25.0.93; sh-mode heredoc triggers incorrectly

[Anders Lindgren <andlind@gmail.com>]

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In some cases, the "heredoc" system of sh-mode triggers on things that
aren't heredocs.

Steps to repeat:

    Create a file, test.sh, with the following content:
========8<--------
# This is a normal comment
echo "<<HELLO>>"
# This is incorrectly highlighted as a heredoc comment.
echo "This is also incorrectly highlighted as a heredoc comment"
========8<--------
    emacs -Q test.sh

Now, the two last lines are highlighted as a heredoc.

    -- Anders

In GNU Emacs 25.0.93.1 (x86_64-w64-mingw32)
 of 2016-04-23 built on KAEL
Windowing system distributor 'Microsoft Corp.', version 6.1.7601
Configured using:
 'configure --prefix=/tmp/emacs --without-imagemagick 'CFLAGS=-O2
 -fomit-frame-pointer -g0''

Configured features:
XPM JPEG TIFF GIF PNG RSVG SOUND DBUS NOTIFY ACL GNUTLS LIBXML2 ZLIB
TOOLKIT_SCROLL_BARS

Important settings:
  value of $LANG: SVE
  locale-coding-system: cp1252

Major mode: Shell-script

Minor modes in effect:
  sh-electric-here-document-mode: t
  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
  line-number-mode: t
  transient-mark-mode: t

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Setting up indent for shell type sh
Indentation variables are now local.
Indentation setup for shell type sh
Mark set
Quit
Type C-x 1 to delete the help window.

Load-path shadows:
None found.

Features:
(shadow sort mail-extr cl-extra pp thingatpt emacsbug message dired
format-spec rfc822 mml mml-sec password-cache epg epg-config gnus-util
mm-decode mm-bodies mm-encode mail-parse rfc2231 mailabbrev gmm-utils
mailheader sendmail rfc2047 rfc2045 ietf-drums mm-util help-fns
help-mode easymenu cl-loaddefs pcase cl-lib mail-prsvr mail-utils
vc-dispatcher vc-svn sh-script smie executable time-date mule-util
tooltip eldoc electric uniquify ediff-hook vc-hooks lisp-float-type
mwheel dos-w32 ls-lisp disp-table w32-win w32-vars term/common-win
tool-bar dnd fontset image regexp-opt fringe tabulated-list newcomment
elisp-mode lisp-mode prog-mode register page menu-bar rfn-eshadow timer
select scroll-bar mouse jit-lock font-lock syntax facemenu font-core
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 charscript
case-table epa-hook jka-cmpr-hook help simple abbrev 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 w32notify dbusbind w32
multi-tty make-network-process emacs)

Memory information:
((conses 16 96532 11635)
 (symbols 56 20246 0)
 (miscs 48 45 138)
 (strings 32 18893 3179)
 (string-bytes 1 500720)
 (vectors 16 12785)
 (vector-slots 8 443089 4303)
 (floats 8 161 32)
 (intervals 56 291 18)
 (buffers 976 14))

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