Adjust to modern mercurial version definition.

Adjust to modern mercurial version definition.

  hg-stable$ hg log --patch mercurial/util.py
  ...
  changeset:   7640:9626819b2e3d
  user:        Matt Mackall <mpm@selenic.com>
  date:        Sat Jan 10 18:02:38 2009 -0600
  summary:     refactor version code

  diff --git a/mercurial/util.py b/mercurial/util.py
  --- a/mercurial/util.py
  +++ b/mercurial/util.py
  @@ -142,6 +142,14 @@
       """Find the length in characters of a local string"""
       return len(s.decode(_encoding, "replace"))

  +def version():
  +    """Return version information if available."""
  +    try:
  +        import __version__
  +        return __version__.version
  +    except ImportError:
  +        return 'unknown'
  +
   # used by parsedate
  ...
  hg-stable$ hg tags
  ...
  1.2                             7823:11efa41037e2
  1.1.2                           7497:11a4eb81fb4f
  ...