D4324: setdiscovery: use revset for resolving DAG heads in a subset

[Yuya Nishihara]

>   For reasons I didn't investigate, feeding a set with nullrev
>   into the heads() revset resulted in a bunch of tests failing.
>   Filtering out nullrev from the input set fixes things.

That's probably because `heads()` internally calls `parents()`, but
`parents()` doesn't handle null in a way `heads()` can exclude p1 of the
root revision.