bug#35202: 27.0.50; Info-quoted false positives and false negatives

[Mauro Aranda <maurooaranda@gmail.com>]

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Hello.  I've noticed that the regexp of Info-mode-font-lock-keywords
throws some false negatives and positives.

In latest Emacs, do these steps to see the bug.
1) Start Emacs: emacs -Q
2) Go to the Emacs Manual: C-h r
3) Go to the Basic node: m Basic RET
4) Go to the Inserting Text section: 1
5) Search for "which is": C-s which is RET
6) Type: M-x describe-text-properties
Notice that face is Info-quoted, even though point is not in a quote.

This is a false positive.  Under the same section, there's another false
positive: "To see which characters have " is quoted too.

Now to show a false negative, in the same Emacs Manual:
Search for this string C-s ‘’’.  That is: C-x 8 [ followed by C-x 8 ]
followed by C-x 8 ].
The search should get you to the Text Display section.  Move point to
the middle ’, the one that is being quoted, and again type:
M-x describe-text-properties.  It doesn't have the face Info-quoted,
even though it is a quote.

I hope I've described well enough the issue.  Proposed patch to follow.

Best regards,
Mauro.

In GNU Emacs 27.0.50 (build 3, i686-pc-linux-gnu, GTK+ Version 3.18.9)
 of 2019-04-08 built on the-blackbeard
Repository revision: 36dc39bfbf1a307769bd62dbe1311a1935737b51
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.11906000
System Description: Ubuntu 16.04.6 LTS

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Mark saved where search started
Type C-x 1 to delete the help window.
Mark saved where search started
Type C-x 1 to delete the help window.

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GSETTINGS GLIB
NOTIFY INOTIFY ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF
XFT ZLIB TOOLKIT_SCROLL_BARS GTK3 X11 XDBE XIM THREADS PDUMPER LCMS2 GMP

Important settings:
  value of $LANG: en_US.utf8
  value of $XMODIFIERS:
  locale-coding-system: utf-8-unix

Major mode: Info

Minor modes in effect:
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  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:
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backquote threads dbusbind inotify lcms2 dynamic-setting
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Memory information:
((conses 8 76132 6951)
 (symbols 24 6685 1)
 (strings 16 20842 1712)
 (string-bytes 1 613744)
 (vectors 8 10074)
 (vector-slots 4 126132 14196)
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