Don't subtract lower merge node range above the merge

Suppose two nodes U (upper) and L (lower) have been merged. U and all
visited nodes lower than U need to adjust their right subtree sums.
However, this does not apply to any of the nodes above U.
Add the current node's count _itree_count()

The count implementation was incorrect. It now also includes the range
of the node itself in addition to the values cached in node->v.