Fix "no attribute '_get_bugdir'" bug in `be help severity`.

Fix "no attribute '_get_bugdir'" bug in `be help severity`.

Bug and patch submitted by Marien Zwart.  I can't think of any side
effect issues this might raise, so I'm committing it ;).  Excerpts
from Marien's email below.

From: Marien Zwart <...>
Date: Tue, 10 Aug 2010 17:45:06 +0200
Subject: [Be-devel] "be help severity" fails (traceback)

...
In current git (d63ec5fd3eca61ea53d06a96bfb52170111fc25d) "be help
severity" produces the following:

Traceback (most recent call last):
  ...
AttributeError: 'Severity' object has no attribute '_get_bugdir'
...
but I am not sure if this has any unintended side effects, so I'd prefer
not to submit this as a merge request (for someone more familiar with
the code to fix this instead).
...