revset: implement a simple 'foo#generations' expression The result of this expression is equivalent to 'foo#g[:]': all reachable ancestors and descendants of a revset foo. While not very useful functionality on its own, its implementation can be tested and revset.relations dict can be easily used by extensions to add other non-subscript relations. Differential Revision: https://phab.mercurial-scm.org/D8394