prune: Retain the tip of each ref even with date pruning

prune: Retain the tip of each ref even with date pruning

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