core: Write a refs/summary file for archive repositories
core: Write a refs/summary file for archive repositories

This will allow us to retrieve all refs over HTTP easily.

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