Get GoSubDag containing hdrgo and all usrgos and their ancesters.

def _get_pltdag_ancesters(self, hdrgo, usrgos, desc=""):
        """Get GoSubDag containing hdrgo and all usrgos and their ancesters."""
        go_srcs = usrgos.union([hdrgo])
        gosubdag = GoSubDag(go_srcs,
                            self.gosubdag.get_go2obj(go_srcs),
                            relationships=self.gosubdag.relationships,
                            rcntobj=self.gosubdag.rcntobj,
                            go2nt=self.gosubdag.go2nt)
        tot_usrgos = len(set(gosubdag.go2obj.keys()).intersection(self.usrgos))
        return self.ntpltgo(
            hdrgo=hdrgo,
            gosubdag=gosubdag,
            tot_usrgos=tot_usrgos,
            parentcnt=False,
            desc=desc)