hbisect: use a revset for ancestor calculation
Since we have revsets we can be more concise in doing the ancestor calulcation.
Significant commits are all descendent of the topmost good commits.
Test Plan:
python run-tests.py test-bisect*
Differential Revision: https://phab.mercurial-scm.org/D1498
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}