bug#6326: 23.1.95; `vc-diff' sometimes reports wrong diff status

[Markus Triska <markus.triska@gmx.at>]

When I open (in emacs -Q) a file under "git" version control and:

   1) view its log with: C-x v l
   2) open any previous revision, for example: n n n f
   3) save the previous revision under a different name:
      C-x C-w difrev.txt RET
   4) on difrev.txt, run `vc-diff' with: C-x v =

it unexpectedly says: "No changes between working revision and
workfile". It SHOULD say that the file is NOT under version control.
Also other operations like C-x v l should say this but currently don't
(C-x v l instead unexpectedly displays the log of the original file).


In GNU Emacs 23.1.95.5 (i386-apple-darwin8.11.1, X toolkit, Xaw3d scroll bars)
 of 2010-04-10 on mt-computer.lan
Windowing system distributor `The X.Org Foundation', version 11.0.10402000
configured using `configure  '--with-x-toolkit=lucid''

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_GB.UTF-8
  value of $XMODIFIERS: nil
  locale-coding-system: nil
  default enable-multibyte-characters: t