This is a version of ostree_repo_traverse_commit_union that also
remembers where the objects came from, by recording the parent
relationships in a hashtable. This can be used to later find which
commits each object was from, which we want to use in fsck.

This means we can later use various operations to heal the repository
because ostree does not assume all objects are there.

This the begining of a fix for ostreedev#345