bug#11339: 24.1.50; read-{buffer,file-name}-completion-ignore-case fails on non-ascii

[Stephen Berman <stephen.berman@gmx.net>]

-1. cd /tmp; touch bäh bÄh bla
0. emacs -Q
1. M-x customize-option RET read-buffer-completion-ignore-case RET.
   Toggle to "on", set for the current session.
   M-x customize-option RET read-file-name-completion-ignore-case RET.
   Toggle to "on", set for the current session.
2. C-x C-f b TAB => *Completions* buffer contains, as expected: bla bÄh bäh.         
3. Visit files `bla', `bäh' and `bÄh', switch to buffer *scratch* and
   repeat step 2, replacing `C-x C-f' with `C-x b' => same results.
4. C-x C-f bä TAB => Emacs completes the input to "bäh".               
5. TAB => Emacs incorrectly reports: "[Sole completion]".                 
6. C-x C-f bÄ TAB => Emacs completes the input to "bÄh".
7. TAB => Emacs incorrectly reports: "[Sole completion]".                 
8. Switch to buffer *scratch* and repeat step 4-7, replacing `C-x C-f'
   with `C-x b' => same results.


In GNU Emacs 24.1.50.1 (i686-suse-linux-gnu, GTK+ Version 2.24.7)
 of 2012-04-18 on escher
Bzr revision: 107959 cyd@gnu.org-20120418070052-e6bm11h52qja62ue
Windowing system distributor `The X.Org Foundation', version 11.0.11004000
Configured using:
 `configure '--without-toolkit-scroll-bars' 'CFLAGS=-g''

Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: @im=local
  locale-coding-system: utf-8-unix
  default enable-multibyte-characters: t