Return Context Manager for a file-like (stream) object to send a diff.

def send(self, diff):
        """ Return Context Manager for a file-like (stream) object to send a diff. """
        if Store.skipDryRun(logger, self.dryrun)("send %s", diff):
            return None

        (diffTo, diffFrom) = self.toArg.diff(diff)
        self._client.send(diffTo, diffFrom)

        progress = DisplayProgress(diff.size) if self.showProgress is True else None
        return _SSHStream(self._client, progress)