Implement get_missing_parents in terms of _KeyRefs.
This gives _GCGraphIndex a _key_dependencies variable.
Things still fail, because get_missing_parent_inventories() assumes that fileids_altered_by_revision_ids will return file_ids for revisions outside of the actual revisions requested. CHKInventoryRepository actually explicitly eliminates those keys. We may just need a custom get_missing_parent_inventories, but one way or another we have issues.