bug#20568: 24.5; ediff-previous-difference broken with focus enabled

[Dyer, Art]

This is reproducible with emacs -Q.

Start ediff. Any pair of files/buffers will do, as long as they produce multiple
difference regions.

Hit "#f" to focus by regular expression. Enter "."  for both of the regular
expressions, then "y" to match on both buffers. (This results in a "focus" that
actually includes all the diffs, but that is just for simplicity - it is not
necessary in order to reproduce the bug.)

Use "n" a few times to advance through the first few difference regions. Then
try to use "p" to back up; instead of the previous diff you will be positioned
before the first one ("At start of <n> diffs").


In GNU Emacs 24.5.1 (x86_64-w64-mingw32)
 of 2015-04-11 on KAEL
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --prefix=/z/emacs --host=x86_64-w64-mingw32
 --target=x86_64-w64-mingw32 --build=x86_64-w64-mingw32 --with-wide-int
 --with-jpeg --with-xpm --with-png --with-tiff --with-rsvg --with-xml2
 --with-gnutls --with-sound=yes --with-file-notification=yes
 --without-dbus --without-imagemagick 'CFLAGS=-Ofast
 -fomit-frame-pointer -funroll-loops -g0 -pipe' 'LDFLAGS=-static-libgcc
 -static-libstdc++ -static -s -Wl,-s''

Important settings:
  value of $LANG: ENU
  locale-coding-system: cp1252

Major mode: Ediff

Minor modes in effect:
  tooltip-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
  buffer-read-only: t
  line-number-mode: t
  transient-mark-mode: t

Recent messages:
Mark set [2 times]
Computing differences between ediff7896uC8 and ediff7896wUx ...
Buffer A: Processing difference region 0 of 4
Buffer B: Processing difference region 0 of 4
Processing difference regions ... done
Focus on regions that match BOTH regexps, OK? (y or n) y
Will focus on regions that match BOTH regexps
Refining difference region 1 ...
Refining difference region 2 ...
Refining difference region 3 ...

Load-path shadows:
None found.

Features:
(shadow sort gnus-util mail-extr emacsbug message format-spec rfc822 mml
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 ediff-merg ediff-wind ediff-diff
ediff-mult ediff-help ediff-init ediff-util ediff help-mode easymenu
apropos time-date tooltip electric uniquify ediff-hook vc-hooks
lisp-float-type mwheel dos-w32 ls-lisp w32-common-fns disp-table w32-win
w32-vars tool-bar dnd fontset image regexp-opt fringe tabulated-list
newcomment 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 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
w32notify w32 multi-tty emacs)

Memory information:
((conses 16 84913 8382)
 (symbols 56 18931 0)
 (miscs 48 113 158)
 (strings 32 14238 4796)
 (string-bytes 1 399330)
 (vectors 16 10384)
 (vector-slots 8 391786 5409)
 (floats 8 69 471)
 (intervals 56 244 28)
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