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

[Eli Zaretskii <eliz@gnu.org>]

> From: Christoph Ruegge <chrueg@gmail.com>
> Date: Wed, 04 Feb 2015 16:31:31 +0100
> 
> Steps to produce the problem:
> 
> 1) emacs -Q -fn Monospace

I see this with the default font as well.

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

I see this regardless of scroll-bar-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 is in vertical-motion (or its subroutines, I didn't yet
look deeper), and it's a regression in the emacs-24 branch wrt Emacs
24.3.  So we should fix it before the release.