pull-local: Explicitly unref variant rather than relying on floating
pull-local: Explicitly unref variant rather than relying on floating

This shows up as a leak in valgrind; the callee isn't sinking.  In
general through the power of cleanup attributes we can do explicit
cleanup rather than relying on floating refs.

Closes: #410
Approved by: giuseppe

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