stack: when stack base is obsolete, pick any successor, even if at random There are situations when s0 is obsolete and we also cannot pick just one successor for it to use in stack. In such a case, let's pick the "latest" successor from the first set. We're assuming that obsutil.successorssets() returns data in the same order (it should, since it makes sure to sort data internally). Keeping that in mind, while the successor picked for s0 by this code is not based on any sort of sophisticated logic, it should nonetheless be the same every time. This patch is probably not going to completely break anything that was previously working fine, because the previous behavior was to just abort with an exception.