D4314: setdiscovery: use a revset instead of dagutil.descendantset()

[Yuya Nishihara]

>   @yuja: I thought I'd draw your attention to the potential revset parser bug detailed in the commit message

`x :: - y ::` is ambiguous because `::` can be infix/postfix op, and `-` can
be prefix/infix op. The infix `::` just wins as the parser only looks for the
next token (1 look-ahead), and the expression is parsed as `(x :: (- y)) ::`,
which is unfortunately valid.