ostree-repo-traverse: Don't leak floating GVariant
ostree-repo-traverse: Don't leak floating GVariant

ostree_object_name_serialize returns a floating ref, so we need
to sink it before putting in the hashtable.

Closes: #595
Approved by: cgwalters

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