bug#5440: 23.1; buffer-file-format encoding temp buffers not reentrant

[Kevin Ryde]

The way build_annotations() makes `format-annotate-function' use the
same temp buffer " *Format Temp 0*" on each call is not reentrant.  If
TO-FN from the format-alist does anything that writes another buffer
with a buffer-file-format then the buffer which TO-FN is supposed to be
working on is clobbered.

foo.el below illustrates the problem.  Evaluating it writes a /tmp/x
containing

    THIS IS FORMAT ONE
    this is buffer yyy text

where I expected it to be

    THIS IS FORMAT TWO
    hello world

The `message's emitted by my-one-encode and my-two-encode show they get
the same " *Format Temp 0*" buffer.  The latter is what you get in
/tmp/x if taking away the "/tmp/y" blob within `my-two-encode'.

I expect this wouldn't arise often in practice, but it ought to be
reasonably easy to make build_annotations() (or wherever) do better in
the management of temporary buffers.  I expect it's a matter of finding
the right place in the write crunching that temp buffers used can be
killed.

foo.el
Description: application/emacs-lisp

In GNU Emacs 23.1.1 (i486-pc-linux-gnu, GTK+ Version 2.16.5)
 of 2009-09-14 on raven, modified by Debian
configured using `configure  '--build=i486-linux-gnu' '--host=i486-linux-gnu' 
'--prefix=/usr' '--sharedstatedir=/var/lib' '--libexecdir=/usr/lib' 
'--localstatedir=/var/lib' '--infodir=/usr/share/info' 
'--mandir=/usr/share/man' '--with-pop=yes' 
'--enable-locallisppath=/etc/emacs23:/etc/emacs:/usr/local/share/emacs/23.1/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/23.1/site-lisp:/usr/share/emacs/site-lisp:/usr/share/emacs/23.1/leim'
 '--with-x=yes' '--with-x-toolkit=gtk' '--with-toolkit-scroll-bars' 
'build_alias=i486-linux-gnu' 'host_alias=i486-linux-gnu' 'CFLAGS=-DDEBIAN -g 
-O2' 'LDFLAGS=-g' 'CPPFLAGS=''

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_AU
  value of $XMODIFIERS: nil
  locale-coding-system: iso-latin-1-unix
  default-enable-multibyte-characters: t