setdiscovery: use revsets for computing a subset's heads and roots revlogdag.headsetofconnecteds() obtains the set of DAG heads in a given set of revs. revlogdag.inverse() inverts the DAG order and makes headsetofconnecteds() obtain the DAG roots in a given subset. Both of these can be expressed with a revset. Like other patches in this series, revlogdag uses revlog.index and thus doesn't take filtering into account. Revsets do. So there is a chance for regressions with this change. But no tests fail. And I think this code should take filtering into account since hidden changesets shouldn't factor into discovery (unless operating on the hidden repository). Differential Revision: https://phab.mercurial-scm.org/D4321