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.
(binary:application/octet-stream)