setdiscovery: use a revset for finding DAG heads in a subset
The march towards moving away from dagutil continues.
Like other patches moving us away from dagutil, there is the
potential for regressions to occur because revlogdag's
headsetofconnecteds() uses revlog.index, which doesn't take
filtering into account. The revset layer does. But no tests
fail, so we appear to be in the clear.
Differential Revision: https://phab.mercurial-scm.org/D4317
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