revset: rename revsbetween to reachableroots and add an argument
This patch is part of a series of patches to speed up the computation of
revset.revsbetween 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 revsbetween is 10-50x faster and smartlog on is
2x-5x faster.
This patch rename 'revsbetween' to 'reachableroots' and makes the computation of
the full path optional. This will allow graphlog to compute grandparents using
'reachableroots' and remove the need for a dedicated grandparent function.