setdiscovery: pass heads into _updatesample() In preparation for eliminating the use of dagutil. Since _takefullsample() operates on the inverted DAG, it is easier to have the caller pass in the relevant set instead of teaching _updatesample() about when to invert the DAG. We keep the logic identical for now: future commits will remove dagutil. Differential Revision: https://phab.mercurial-scm.org/D4318