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
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