remote-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>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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