repo: Enumerate objects using openat() too, make more efficient
repo: Enumerate objects using openat() too, make more efficient

This drops several calls to malloc()/g_object_new() per object
enumeration.  Just a followup to using openat() and friends in other
places.

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