prune: Default to deleting loose objects, code cleanup
prune: Default to deleting loose objects, code cleanup

We're getting closer to matching 'git gc'.

resin-io/ostree,sujitfulse/ostree,resin-io/ostree,sujitfulse/ostree,gatispaeglis/ostree,GNOME/ostree,leandrosansilva/ostree,gatispaeglis/ostree,leandrosansilva/ostree,GNOME/ostree,gatispaeglis/ostree,gatispaeglis/ostree,GNOME/ostree,leandrosansilva/ostree,leandrosansilva/ostree,GNOME/ostree,sujitfulse/ostree,resin-io/ostree,sujitfulse/ostree,sujitfulse/ostree,resin-io/ostree,GNOME/ostree,leandrosansilva/ostree,gatispaeglis/ostree,gatispaeglis/ostree,resin-io/ostree