COMMITS=$(git cat-file --batch-check='%(objectname) %(objecttype)' --batch-all-objects | grep commit | cut -f1 -d" ")

for COMMIT in $COMMITS; do
    # My first parent's parents has to be the same as my second parent's parent.
    if [ "$(git rev-parse --verify -q $COMMIT^1^)" = "$(git rev-parse --verify -q $COMMIT^2^)" ]; then
        return 0
    fi
done

return 1