repo: Port metadata writing code to fd-relative
repo: Port metadata writing code to fd-relative

It was the last thing referencing `self->objects_dir`.

Closes: #432
Approved by: giuseppe

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