deltas: Fix regression in ostree_repo_static_delta_execute_offline
deltas: Fix regression in ostree_repo_static_delta_execute_offline

xdg-app passed this a filename directly, and in this case it should be
used as is. This regressed to always look for "superblock" in the same
directory as the passed in filename.

https://bugzilla.gnome.org/show_bug.cgi?id=762617

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