revset: improve roots revset performance Previously we would iterate over every item in the subset, checking if it was in the provided args. This often meant iterating over every rev in the repo. Now we iterate over the args provided, checking if they exist in the subset. On a large repo this brings setting phase boundaries (which use this revset roots(X:: - X::Y)) down from 0.8 seconds to 0.4 seconds. The "roots((tip~100::) - (tip~100::tip))" revset in revsetbenchmarks shows it going from 0.12s to 0.10s, so we should be able to catch regressions here in the future. This actually introduces a regression in 'roots(all())' (0.2s to 0.26s) since we're now using spansets, which are slightly slower to do containment checks on. I believe this trade off is worth it, since it makes the revset O(number of args) instead of O(size of repo).