bug#19769: 25.0.50; Incorrect beginning of line in visual-line-mode

[Christoph Ruegge <chrueg@gmail.com>]

Steps to produce the problem:

1) emacs -Q -fn Monospace

2) In the scratch buffer, disable scroll-bar-mode and enable
visual-line-mode.

3) Type, on a single line: 'aaa[...]aa bbbb cccc', where the number of 'a's is
chosen such that
- the 'b's appear at the end of the line,
- the 'c's are wrapped to the next line by visual-line-mode,
- one more 'a' would make the 'b's also wrap to the next line.

4) Position the cursor after the 'c's and invoke beginning-of-visual-line.

Result: The cursor is at the first 'b', but should be at the first 'c'.

The problem also occurs with other fonts instead of Monospace passed to
the -fn option, but not when omitting the option.

In GNU Emacs 25.0.50.11 (x86_64-unknown-linux-gnu, GTK+ Version 3.14.8)
 of 2015-02-04 on io
Repository revision: 5b83f03725e713c268ec1417ca36f8d3b65905d4
Windowing system distributor `The X.Org Foundation', version 11.0.11603000
System Description:	Arch Linux

Configured using:
 `configure --prefix=/home/cs/.local --with-x-toolkit=gtk'

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GCONF GSETTINGS
NOTIFY ACL GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB

Important settings:
  value of $LC_MONETARY: en_DK.UTF-8
  value of $LC_NUMERIC: en_DK.UTF-8
  value of $LC_TIME: en_DK.UTF-8
  value of $LANG: en_DK.UTF-8
  value of $XMODIFIERS: @im=fcitx
  locale-coding-system: utf-8-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

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Making completion list...

Load-path shadows:
None found.

Features:
(shadow sort gnus-util mail-extr emacsbug message dired format-spec
rfc822 mml easymenu mml-sec mm-decode mm-bodies mm-encode mail-parse
rfc2231 mailabbrev gmm-utils mailheader sendmail rfc2047 rfc2045
ietf-drums mm-util help-fns mail-prsvr mail-utils time-date tooltip
eldoc electric uniquify ediff-hook vc-hooks lisp-float-type mwheel x-win
x-dnd 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 cham georgian utf-8-lang misc-lang vietnamese
tibetan thai tai-viet lao korean japanese hebrew greek romanian slovak
czech european ethiopic indian cyrillic chinese 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 make-network-process dbusbind
gfilenotify dynamic-setting system-font-setting font-render-setting
move-toolbar gtk x-toolkit x multi-tty emacs)

Memory information:
((conses 16 75152 3276)
 (symbols 48 18087 0)
 (miscs 40 40 124)
 (strings 32 11065 4218)
 (string-bytes 1 300732)
 (vectors 16 9359)
 (vector-slots 8 386511 16301)
 (floats 8 71 62)
 (intervals 56 190 0)
 (buffers 976 12)
 (heap 1024 46726 1043))