revset: remove grandparent by using reachableroots This patch is part of a series of patches to speed up the computation of revset.reachableroots by introducing a C implementation. The main motivation is to speed up smartlog on big repositories. At the end of the series, on our big repositories the computation of reachableroots is 10-50x faster and smartlog on is 2x-5x faster. Before this patch, we had a custom computation for grandparent that was very close to the idea of reacheablerooots. This patch expresses grandparent with reachableroots to reduce the amount of code.