Allow symlinks in bugdir directories.

Allow symlinks in bugdir directories.

This allows you to do things like:

  $ mkdir -p /tmp/joint-repo/.be
  $ cd /tml/joint-repo/.be
  $ ln -s ~/src/be/.be/version
  $ ln -s ~/src/be/.be/bea86499-824e-4e77-b085-2d581fa9ccab/
  $ ln -s ~/src/BEurtle/.be/7017b289-f207-4e39-9746-f58323404eba/
  $ be list

without crashing with a:

  Traceback (most recent call last):
    File ".../libbe/storage/base.py", line 316, in children
      return self._children(*args, **kwargs)
    File ".../libbe/storage/vcs/base.py", line 820, in _children
      path = self.path(id, revision, relpath=False)
    File ".../libbe/storage/vcs/base.py", line 721, in path
      path = self._cached_path_id.path(id)
    File ".../libbe/storage/vcs/base.py", line 280, in path
      raise InvalidID(uuid)
  libbe.storage.base.InvalidID: 7017b289-f207-4e39-9746-f58323404eba in revision None

Currently it only lists the first bug directory it comes across, but
after this patch, it doesn't crash ;).