bug#20926: 25.0.50; Quotation marks always display as hard quotes

[Artur Malabarba <bruce.connor.am@gmail.com>]

1. Start emacs -Q
2. go to *scratch* buffer
3. Type C-x RET 8 LEFT SINGLE QUOTATION MARK

At first sight, it seems that a hard quote gets inserted
instead. Examining it with `describe-char' (below) reveals that it is indeed a
round quote, but the display table is hiding it.
Note that the display table hides it even on the Help buffer itself!

The same happens for double quotes. And it's not exlusive to elisp
mode (I tested fundamental and text modes as well).
I haven't managed to follow all of the quote-gate conversation, but
I'm assuming this is a bug, as I can't think of a situation where it
would be right to mask a double quote the user has explicitly typed.

---

             position: 411 of 3041 (13%), column: 0
            character: ‘ (displayed as ‘) (codepoint 8216, #o20030, #x2018)
    preferred charset: unicode (Unicode (ISO10646))
code point in charset: 0x2018
               script: symbol
               syntax: . which means: punctuation
             category: .:Base, c:Chinese, h:Korean, j:Japanese
             to input: type "C-x 8 RET 2018" or "C-x 8 RET LEFT SINGLE
QUOTATION MARK"
          buffer code: #xE2 #x80 #x98
            file code: #xE2 #x80 #x98 (encoded by coding system utf-8-emacs)
              display: by display table entry [?'] (see below)

The display table entry is displayed by these fonts (glyph codes):
': xft:-unknown-Ubuntu
Mono-normal-normal-normal-*-13-*-*-*-m-0-iso10646-1 (#x0A)

Character code properties: customize what to show
  name: LEFT SINGLE QUOTATION MARK
  old-name: SINGLE TURNED COMMA QUOTATION MARK
  general-category: Pi (Punctuation, Initial quote)
  decomposition: (8216) ('‘')

There are text properties here:
  fontified            t

In GNU Emacs 25.0.50.1 (x86_64-unknown-linux-gnu, GTK+ Version 2.24.23)
 of 2015-06-01 on it055607
Windowing system distributor `The X.Org Foundation', version 11.0.11501000
System Description: Ubuntu 14.04.2 LTS

Configured features:
XPM JPEG TIFF GIF PNG SOUND GSETTINGS NOTIFY GNUTLS LIBXML2 FREETYPE XFT
ZLIB

Important settings:
  value of $LANG: en_GB
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: iso-latin-1-unix

Major mode: Lisp Interaction

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
  line-number-mode: t