repo: Fix major performance regression with --scan-hardlinks
repo: Fix major performance regression with --scan-hardlinks

gnome-continuous uses the ostree_repo_scan_hardlinks() mode to
avoid re-checksumming everything.  However, when I ported the commit
code to use openat() and friends, this optimization was lost.

Re add it.  The difference is about 15s versus 5 minutes.

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