core: Use linkat() rather than link() in threaded checkouts
core: Use linkat() rather than link() in threaded checkouts

This seems to work around a likely Linux kernel VFS bug, where I
randomly see ENOENT on link() when we *definitely* called mkdir() at
an earlier point in time.

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