'Create and return a new endpoint which will try to open a new channel on
an existing SSH connection and run a command over it.  It will B{not}
close the connection if there is a problem executing the command or
after the command finishes.
@param connection: An existing connection to an SSH server.
@type connection: L{SSHConnection}
@param command: See L{SSHCommandClientEndpoint.newConnection}\'s
C{command} parameter.
@type command: L{bytes}
@return: A new instance of C{cls} (probably
L{SSHCommandClientEndpoint}).'

@classmethod
def existingConnection(cls, connection, command):

    helper = _ExistingConnectionHelper(connection)
    return cls(helper, command)
