setdiscovery: use a revset instead of dagutil.descendantset()
This is the only use of descendantset() in the repo.
Strictly speaking, the revset behaves slightly differently than
dagutil. The reason is that dagutil is using revlog.index for
DAG traversal and this data structure isn't aware of visibility /
filtering. So it can operate on revisions it shouldn't operate on.
But our test coverage of this code is pretty comprehensive and
this change causes no tests to fail. So I think we are good.
Also, the revset parser failed to parse `%ld:: - %ld::`, hence
the use of descendants(). I'm not sure if that is a feature or
a bug.
Differential Revision: https://phab.mercurial-scm.org/D4314
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