repo: Fix object list keys ownership

repo: Fix object list keys ownership

ostree_object_name_serialize returns a floating ref, so sink it before
adding it to the hash table so it can properly be freed later when the
hash table is destroyed.

This is particularly a problem for pygobject, which sinks the refs on
variants as it marshals them to native python types. If the ref isn't
already sunk, then the ref count won't increase and a critical warning
will be raised when both the hash table and pygobject try to unref it.

Closes: #635
Approved by: cgwalters