revsetbenchmark: add revset with lazyset subtraction The added revset is used by obsolescence and currently results in recursion in __contains__ between 2 lazysets. We should have coverage of this revset.

debugrevlog: add chainlen column to --dump output

debugdag: stop wrongly sorting parents The dag being dumped is not in a format that allows us to reconstruct the original dag as the parent revisions are normalised.