fix panic in accumulateCommits when the operator returns nil

This can happen because @par and @chn simply return the raw list of parents/children, which can now be sparse. A better (but longer) fix would be for parents() and children() to use the iterator instead.

I mistakenly pushed this commit to your repository rather than my own! If you prefer this simple patch over the slightly more complex approach using the iterators, then accept this merge request and delete the branch I created for it. Otherwise, deny this merge request and delete the branch even though it isn’t merged.