bug#2982: 23.0.92; `vc-diff' behaves inconsistently with git

When I do:

   $ mkdir test
   $ cd test
   $ git init
   $ touch a.txt
   $ git add a.txt
   $ git commit -m "initial revision"
   $ emacs -Q a.txt

then pressing "C-x v =" shows:

   No changes between working revision and workfile

in the echo area. However, when I do:

   M-! touch a.txt RET

then "C-x v =" shows:

   Finding changes in /Users/mt/test/a.txt...done

and a *vc-diff* buffer remains visible, showing:

   diff --git a/a.txt b/a.txt

although, as before, a.txt is unchanged. In a new Emacs instance,
"C-x v =" again behaves like the first time. git version: 1.6.2.

In GNU Emacs 23.0.92.2 (i386-apple-darwin9.6.1, GTK+ Version 2.14.7)
 of 2009-04-05 on mt-imac.local
Windowing system distributor `The X.Org Foundation', version 11.0.10402000
configured using `configure  '--with-tiff=no''

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: nil
  locale-coding-system: utf-8-unix
  default-enable-multibyte-characters: t