ediff-select-hook is called twice when a difference is selected

[Klaus Zeitler <kzeitler@lucent.com>]

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In GNU Emacs 21.3.1 (sparc-sun-solaris2.8, Motif Version 2.1.0)
 of 2003-07-10 on sfsws4
configured using `configure  --with-gcc --prefix=/vol/freeware/SunOS-5.8/emacs/21.3 --sharedstatedir=/vol/freeware/SunOS-5.8/emacs/21.3/share --with-x-toolkit=motif --x-includes=/usr/openwin/include:/usr/local/gnu/include --x-libraries=/usr/openwin/lib:/usr/local/gnu/lib --with-pop'
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:

I've tried to use ediff-select-hook to execute some specific action when I
select a new difference.
When I select the next/previous difference ediff-select-hook is called twice,
AFAICT it seems as if it's called in ediff-unselect-and-select-difference for
the new and the old diff.

The following code snippet executed in *scratch* buffer

--- snip ---
(setq select-calls 0)
(defun ediff-test-select-hook ()
  (setq select-calls (1+ select-calls))
  (message "ediff-test-select-hook called %d times" select-calls))
(add-hook 'ediff-select-hook 'ediff-test-select-hook)
--- snip ---

shows after next/previous-difference in *Messages* buffer:
ediff-test-select-hook called 1 times
ediff-test-select-hook called 2 times


I got around this by modifying (in ediff-unselect-and-select-difference):
	  (or (eq flag 'unselect-only)
	      (ediff-select-difference n))
to:
	  (or (eq flag 'unselect-only)
	      (let (ediff-select-hook)
		(ediff-select-difference n)))

that may not be a good solution, but it works for me

Recent input:
<down-mouse-1> <mouse-movement> <mouse-1> <down-mouse-2> 
<mouse-2> <up> <up> <up> <up> <up> <up> <left> C-j 
<down> <down> <down> <down> <down> <left> C-j <down> 
<down> <down> <left> C-j <escape> x e d i f f - f i 
l SPC <return> t s t / e d 1 <return> <up> <backspace> 
2 <return> n q y C-x b * M SPC <return> <help-echo> 
<help-echo> <help-echo> <help-echo> <menu-bar> <help-menu> 
<report-emacs-bug>

Recent messages:
Buffer A: Processing difference region 0 of 1
Buffer B: Processing difference region 0 of 1
Processing difference regions ... done
Refining difference region 1 ...
 [2 times]
ediff-test-select-hook called 1 times
ediff-test-select-hook called 2 times
 [2 times]
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