bug#17969: 24.3.92; C-p does not move the cursor for some scale text

[YAMAMOTO Mitsuharu]

Steps to reproduce:

1. $ emacs -Q -D &
2. Type the following expression and hit C-j in the *scrath* buffer.

  (progn
    (text-scale-set 6)
    (insert (make-string 9 ?\t) "aaaa\n"))

   In my environment, the screen looks like the attached screenshot.

3. Hit C-p twice.

Result:

The first C-p moves the cursor upward, but for the second one (the
cursor stays at the same position).

Unfortunately, some adjustment seems to be necessary for the
reproducing recipe depending on the environment.  The above one is for
the GTK+ build with the xft font backend on OS X 10.9.  For the Mac
port, I had to use the following one, for example:

  (progn
    (text-scale-set 5)
    (insert (make-string 10 ?\t) "aaaaaa\n"))

                                     YAMAMOTO Mitsuharu
                                mituharu@math.s.chiba-u.ac.jp

In GNU Emacs 24.3.92.1 (x86_64-apple-darwin13.2.0, GTK+ Version 3.12.2)
 of 2014-07-07 on YAMAMOTO-no-iMac.local
Repository revision: 117359 rgm@gnu.org-20140706235852-o9oxnu7jb525f42v
Windowing system distributor `The X.Org Foundation', version 11.0.11406000
Configured using:
 `configure --without-imagemagick LDFLAGS=-L/opt/local/lib
 CPPFLAGS=-I/opt/local/include'