Fixed shortname -> bugname in becommands/show.py.

Fixed shortname -> bugname in becommands/show.py.

Fixes
  wking@thor:be.target-as-bug$ be show 22b:7
  Traceback (most recent call last):
    File "/home/wking/bin/be", line 65, in <module>
      sys.exit(cmdutil.execute(args[0], args[1:]))
    File "/home/wking/src/fun/be/be.target-as-bug/libbe/cmdutil.py", line 87, in execute
      restrict_file_access=restrict_file_access)
    File "/home/wking/src/fun/be/be.target-as-bug/becommands/show.py", line 82, in execute
      print output(args, bd, as_xml=options.XML, with_comments=options.comments)
    File "/home/wking/src/fun/be/be.target-as-bug/becommands/show.py", line 174, in output
      lines.append(comment.string(shortname=shortname))
  NameError: global name 'shortname' is not defined