create tomporary files with sftp

[Markus]

Using ubuntu lucid ppa: 0.6.13-0ubuntu1~lucid1 with sftp.

when a transfer with put_sftp fails in the middle of it, there's an unfinished file left on the backend (which duplicity treats as a complete one on next run). This can be avoided simply:

- upload file with a temporary filename ignored by list()
- rename the file after a successful upload

i simply accomplished this changing backends/sshbackend.py put_sftp():

    def put_sftp(self, source_path, remote_filename = None):
        """Use sftp to copy source_dir/filename to remote computer"""
        if not remote_filename:
            remote_filename = source_path.get_filename()

        // changed to upload to a temp name and then finally rename
        commands = ["put \"%s\" \"%s.%s.part\"" %
                    (source_path.name, self.remote_prefix, remote_filename),
                    "rename \"%s.%s.part\" \"%s%s\"" %
                    (self.remote_prefix, remote_filename,self.remote_prefix, remote_filename)]
        commandline = ("%s %s %s" % (globals.sftp_command,
                                     globals.ssh_options,
                                     self.host_string))
        self.run_sftp_command(commandline, commands)

Now the file first gets created with a '.' as a prefix and a '.part' suffix. it is ignored by the "ls -1" in list(), so duplicity does not get confused. when uploaded - at least when the connection does not drop and the PUT returns to 'sftp>' - the file gets renamed to the desired name.

same should be done for put_scp, i think.

this helps from silently corrupted incremental backup sets when connection to the backend drops in the middle of the transfer and next backup starts.