libostree: Adjust `cleanup_ref_prefix` to use ostree_repo_list_refs_ext
libostree: Adjust `cleanup_ref_prefix` to use ostree_repo_list_refs_ext

Signed-off-by: Giuseppe Scrivano <20002b7b24c75512093a71a58285826d5c504fca@redhat.com>

GNOME/ostree,GNOME/ostree,GNOME/ostree,gatispaeglis/ostree,gatispaeglis/ostree,GNOME/ostree,GNOME/ostree,gatispaeglis/ostree,gatispaeglis/ostree,gatispaeglis/ostree,gatispaeglis/ostree