bug#2354: 23.0.90; Emacs fails to detect utf-8 encoding with language environment Latin-1

[David Engster <deng@randomsample.de>]

This is what I believe to be a regression in CVS Emacs since the
23.0.90 pretest. I'm using a fresh CVS checkout from 2009-02-17,
compiled with 'make bootstrap'.

You can reproduce it as follows:

1. emacs -Q
2. M-x set-language-environment RET Latin-1 RET
3. In some buffer write:

 (ucs-insert "2500")

4. Eval it, so that the unicode character is inserted into the buffer.
5. Save the file and choose utf-8 as encoding.
6. Kill the buffer.
7. Load the file you just saved.

Result: Emacs displays "â\224\200" for the unicode character.

Expected behaviour: Emacs should detect utf-8 encoding and display
correct character.

Please note that this has worked without problems with the Emacs
23.0.90 pretest, so it must be due to some change(s) since then in CVS.

In GNU Emacs 23.0.90.1 (i686-pc-linux-gnu, GTK+ Version 2.12.11)
 of 2009-02-17 on void
Windowing system distributor `The X.Org Foundation', version 11.0.10402000
configured using `configure  '--prefix=/usr/local/emacs''

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: nil
  value of $XMODIFIERS: nil
  locale-coding-system: nil
  default-enable-multibyte-characters: t

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  tool-bar-mode: t
  mouse-wheel-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  global-auto-composition-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 r e p o <tab> r <tab> C-g M-x s e t - l a n <tab> 
<return> L a t i n w <backspace> - w <return> <backspace> 
1 <return> M-x r e p o <tab> r <tab> <return>

Recent messages:
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