bug#13108: 24.3.50; previous-line sometimes moves up two lines instead of just one

[Ami Fischman]

Repro recipe:

gmacs -Q
M-x linum-mode RET  (in the *Scratch* buffer)
ctrl-p (moves up one line; yay)
ctrl-p (moves up two lines; boo!)

This is with emacs built at c6aa556227134725a812bb71c5b2de42e4f135a6


In GNU Emacs 24.3.50.1 (x86_64-unknown-linux-gnu, X toolkit, Xaw scroll bars)
 of 2012-12-06 on fischman-linux.<redacted>
Windowing system distributor `The X.Org Foundation', version 11.0.11103000
System Description:     Ubuntu 12.04.1 LTS

Configured using:
 `configure '--prefix=/usr/gmacs-20121206'
 '--mandir=${prefix}/share/man' '--infodir=${prefix}/share/info'
 '--with-x-toolkit=lucid' '--with-xpm' '--with-jpeg' '--with-tiff'
 '--with-gif' '--with-png' '--with-x'
 '--program-transform-name=s/emacs/gmacs/g' '--without-dbug'
 '--without-gconf' 'CFLAGS=-Wall -g -O2'
 'LDFLAGS=-Wl,-Bsymbolic-functions -Wl,-z,relro'
 'CPPFLAGS=-D_FORTIFY_SOURCE=2' 'LIBS=-lfontconfig -lfreetype''

Important settings:
  value of $LC_ALL: 
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix
  default enable-multibyte-characters: t

Major mode: Lisp Interaction

Minor modes in effect:
  linum-mode: t
  tooltip-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
  transient-mark-mode: t

Recent input:
M-x l i n u <tab> <return> <up> <up> M-x r e p o r 
t <tab> <return>

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Linum mode enabled

Load-path shadows:
None found.

Features:
(shadow sort gnus-util mail-extr emacsbug message 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 mail-prsvr mail-utils linum time-date tooltip ediff-hook
vc-hooks lisp-float-type mwheel x-win x-dnd tool-bar dnd fontset image
regexp-opt fringe tabulated-list newcomment lisp-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 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 dynamic-setting
system-font-setting font-render-setting x-toolkit x multi-tty emacs)