Description of problem:
The recent duplicity update caused my backup scripts to fail. This was due to not having a username specified in the scp://  based destination url.

Previously I had scp://hostname/path and this now fails. I have had to change it to scp://user@hostname/path.

My "hostname" is actually a ssh config entry pointing to the real host and I have specified the correct user there. I see no need to specify it again.

Version-Release number of selected component (if applicable):
duplicity-0.4.12-1.fc9.i386

How reproducible:
Always

Steps to Reproduce:
1. Try and run duplicity /source_path/ scp://remotehost/target_path
2.
3.
  
Actual results:
Fails with;
Traceback (most recent call last):
  File "/usr/bin/duplicity", line 482, in <module>
    with_tempdir(main)
  File "/usr/bin/duplicity", line 477, in with_tempdir
    fn()
  File "/usr/bin/duplicity", line 405, in main
    action = commandline.ProcessCommandLine(sys.argv[1:])
  File "/usr/lib/python2.5/site-packages/duplicity/commandline.py", line 464, in ProcessCommandLine
    backup, local_pathname = set_backend(args[0], args[1])
  File "/usr/lib/python2.5/site-packages/duplicity/commandline.py", line 374, in set_backend
    backend1, backend2 = backends.get_backend(arg1), backends.get_backend(arg2)
  File "/usr/lib/python2.5/site-packages/duplicity/backends.py", line 89, in get_backend
    return backend_class(pu)
  File "/usr/lib/python2.5/site-packages/duplicity/backends.py", line 334, in __init__
    self.host_string = parsed_url.username + "@" + parsed_url.hostname
TypeError: unsupported operand type(s) for +: 'NoneType' and 'str'


Expected results:
Performs backup.

Additional info: