Add ParamikoSSHVendor.
Rename SSHVendor.connect_ssh to SSHVendor.run_command.
Add example showing the use of diff.
Add 'dulwich diff' command.
Handle case that client does not send wanted refs.

Determine_wants can return empty dict if client
does not send wanted refs. Avoid stack trace.

Fixes #106

Signed-off-by: Jelmer Vernooij <jelmer@samba.org>
Actually, enforce determine_wants() returns an empty list.