with ediff, which-function-mode does not update the function in mode-line

[Philippe Waroquiers]

In GNU Emacs 21.1.2 (hppa1.1-hp-hpux11.00, X toolkit)
 of 2001-10-26 on rhino
configured using `configure  hppa1.1-hp-hpux11.00 --with-gcc=no --with-x11r6 
--x-libraries=/usr/lib/X11R6:/usr/contrib/X11R6/lib --with-x-toolkit=lucid 
--with-xpm --prefix=/opt/contrib/emacs --exec-prefix=/opt/contrib/emacs/21.1 
--mandir=/opt/contrib/man'
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: C
  locale-coding-system: nil
  default-enable-multibyte-characters: t

Please describe exactly what actions triggered the bug
and the precise symptoms of the bug:

start emacs with:
 emacs --no-init-file --no-site-file
enable which-function-mode:
 M-x which-function-mode
find the files x.c and y.c given below.
position the cursor in both buffers inside the main function.
At that point, mode-line should show that both buffers are in main functions.
run ediff using the menu:
Tools -> Compare -> Two buffers
(give x.c and y.c as buffer).
2 differences should be reported.
go to the second difference (using n command in the ediff control window).
Some parts of the mode line is updated (e.g. the line nr) but
the function name is not.

----- cut here x.c 
int
main ()
{
  ; /* this is main x */
}

int 
p ()
{
  int i;
  printf ("abcd"); /* this is p x */
}

----- cut here y.c
int
main ()
{
  ; /* this is main y */
}

int 
p ()
{
  int i;
  printf ("abcd"); /* this is p y */
}