traverse: Use g_hash_table_add
traverse: Use g_hash_table_add

And "move semantics" via `g_steal_pointer()`.  Just a minor code
cleanup I noticed when I was hunting for a leak, which ended up being
elsewhere.

Closes: #598
Approved by: jlebon

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