reachableroots: add a 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. This patch introduces a C implementation for reachableroots following closely the Python implementation but optimized by using C data structures.