patchremote-bzr: fix cloning of non-listable repos

> Commit 95b0c60 (remote-bzr: add support for bzr repos) introduced a
> regression by assuming all bzr remote repos are listable, but they are
> not.
>
> If they are not listable they are basically useless, so let's assume
> there is no bzr repo.
>
> Reported-by: Thorsten Kranzkowski <dl8bcu@dl8bcu.de>
> Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
> ---
>
> We want this on master for v1.8.3.

>
>  contrib/remote-helpers/git-remote-bzr | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/contrib/remote-helpers/git-remote-bzr b/contrib/remote-helpers/git-remote-bzr
> index b295dd4..ad42317 100755
> --- a/contrib/remote-helpers/git-remote-bzr
> +++ b/contrib/remote-helpers/git-remote-bzr
> @@ -840,6 +840,9 @@ def get_repo(url, alias):
>  
>      try:
>          repo = origin.open_repository()
> +        if not repo.user_transport.listable():
> +            # this repository is not usable for us
> +            raise bzrlib.errors.NoRepositoryPresent(repo.bzrdir)
>      except bzrlib.errors.NoRepositoryPresent:
>          # branch