bug#2157: 23.0.90; bzr support: vc-diff doesn't work with prefix argument on unchanged files

[Torsten Bronger <bronger@physik.rwth-aachen.de>]

If I visit an unchanged file under bzr revision control, i.e. the modeline says
"Bzr-" (NOT "Bzr:"), I can't use C-u C-x v =, i.e. vc-diff but with explicit
revision parameters.  The error message is:

vc-bzr-previous-revision: Wrong type argument: stringp, nil



In GNU Emacs 23.0.90.1 (x86_64-unknown-linux-gnu)
 of 2009-02-02 on wilson
Windowing system distributor `The Cygwin/X Project', version 11.0.60899901
configured using `configure  '--prefix=/usr/local/' '--mandir=/usr/local/share/man/' '--infodir=/usr/local/share/info/' '--with-x-toolkit=no' '--with-xpm' '--with-jpeg' '--with-tiff' '--with-gif' '--with-png' '--with-x''

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: de_DE.utf8
  value of $XMODIFIERS: nil
  locale-coding-system: utf-8-unix
  default-enable-multibyte-characters: t

Major mode: Python

Minor modes in effect:
  diff-auto-refine-mode: t
  global-auto-revert-mode: t
  savehist-mode: t
  display-time-mode: t
  desktop-save-mode: t
  mouse-wheel-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  global-auto-composition-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t