graft: use revset for intersecting with ancestor set
This addresses a TODO added in
a1381eea7c7d (graft: do not use
`.remove` on a smart set (regression), 2014-04-28).
Differential Revision: https://phab.mercurial-scm.org/D7806
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.