This fixes the code in ConnGraph.optimizeForestEdges() that computes the resulting size after combining two Euler tour trees into one. Technically, the old behavior was still correct, but only by accident; this change is still kind of a bug fix.
This optimizes the process of searching for a replacement edge to refrain from pushing forest edges down when a tree does not have any same-level non-forest edges. If a tree doesn't have any such edges, then there definitely isn't a replacement edge at that level, so we can avoid doing extra work.
Interestingly, with this optimization, ConnGraph reduces to a single Euler tour forest if the graph is a forest, with addEdge and removeEdge taking O(log N) (non-amortized) time with high probability.
