bug#10211: 24.0.92; Case folding on file name completion fails with umlauts

[Stephen Berman]

1. cd /tmp
2. touch aa Aa äa Äa
3. emacs -Q
4. M-x customize-option RET read-file-name-completion-ignore-case RET,
   toggle to `on' and save for the current session.
5. `C-x C-f /tmp/a TAB' shows both aa and Aa as completions.
6. `C-x C-f /tmp/ä TAB' shows äa as the sole completion.

(I recall reading a similar report but I couldn't find it by searching
for "completion", "umlaut", "case", or "ascii".)

In GNU Emacs 24.0.92.1 (i686-suse-linux-gnu, GTK+ Version 2.24.7)
 of 2011-12-03 on escher
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