reachableroots: default to the C implementation 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, reachableroots was computed in pure Python by default. This patch makes the C implementation the default and provides a speedup for reachableroots.