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
#require docutils gettext
$ $TESTDIR/check-gendoc de
checking for parse errors
Die Dateien werden dem Projektarchiv beim n\xc3\xa4chsten \xc3\x9cbernehmen (commit) hinzugef\xc3\xbcgt. Um dies vorher r\xc3\xbcckg\xc3\xa4ngig zu machen, siehe:hg:`forget`. (esc)
warning: please have a space before :hg: