revset: improve performance of _generatorset.__contains__ (issue 4201) _generatorset.__contains__ and __contains__ from child classes were calling into __iter__ to look for values. Since all previously-encountered values from the generator were cached and checked in __contains__ before this iteration, __contains__ was effectively performing iteration busy work which could lead to an explosion of redundant work. This patch changes __contains__ to be more intelligent. Instead of looking at all values via __iter__, __contains__ will instead go straight to "new" values from the underlying generator. On a clone of the Firefox repository with around 200,000 changesets, this patch decreases the execution time of the revset '::(200067)::' from ~100s to ~4s on the author's machine. Rebase operations (which use the aforementioned revset), speed up accordingly.