I hit an error with [CAHC](https://wiki.centos.org/SpecialInterestGroup/Atomic/Devel) where we were doing time-based pruning. `ostree summary -u` started failing, and it took me a bit to realize it was because we were pruning even the tip of old branches, which I was not at all expecting, and I don't think users will too. Perhaps in the future we could add some sort of --prune-ref-tips or something if people wanted it, but I doubt it. Closes: #445 Approved by: jlebon