changelog: add way to call the reachableroots 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 allows us to call the new C implementation of reachableroots from python by creating an entry point in the changelog class.