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
% creating repo
% add and commit
C test.py
% change
M test.py