merge-upstream doesn't use transport to get the upstream tarball

Using merge-upstream doesn't use a transport to get the new upstream version,
so it requires an extra step.

bzr: ERROR: exceptions.IOError: [Errno 2] No such file or directory: u'http://httplib2.googlecode.com/files/httplib2-0.4.0.tar.gz'

Traceback (most recent call last):
  File "/home/jw2328/devel/bzr/bzr.dev/bzrlib/commands.py", line 857, in run_bzr_catch_errors
    return run_bzr(argv)
  File "/home/jw2328/devel/bzr/bzr.dev/bzrlib/commands.py", line 797, in run_bzr
    ret = run(*run_argv)
  File "/home/jw2328/devel/bzr/bzr.dev/bzrlib/commands.py", line 499, in run_argv_aliases
    return self.run(**all_cmd_args)
  File "/home/jw2328/.bazaar/plugins/builddeb/__init__.py", line 460, in run
    repack_tarball(tarball, dest_name, target_dir=orig_dir)
  File "/home/jw2328/.bazaar/plugins/builddeb/repack_tarball.py", line 75, in repack_tarball
    f = open(orig_name)
IOError: [Errno 2] No such file or directory: u'http://httplib2.googlecode.com/files/httplib2-0.4.0.tar.gz'