Even if you're deduplicating commits / data internally, you can tell that that commit is not present in that repo as it is not an ancestor to any ref in that re

Even if you're deduplicating commits / data internally, you can tell that that commit is not present in that repo as it is not an ancestor to any ref in that repository. (You might argue that determining what refs contain a commit is potentially expensive, perhaps, but GitHub already does this, so I'd argue that it's not that expensive.)