revsetbenchmarks: allow running multiple variants per revset The current benchmarks were only testing the whole iteration. This is suboptimal because some changes are meaningful for things like first result, minimum or sorting. We introduce a "variants" feature that let you systematically add some variants to all revsets tested. A typical variants value would be 'plain,min,last,sort'. When testing 'all()' it will also provide testing for: - all() - min(all()) - last(all()) - sort(sort) and output: plain min last sort 0) 0.034568 0.037857 0.000074 0.034238 1) 0.011358 32% 0.020181 53% 0.000080 108% 0.011405 33% Using revsets (who hit the API) instead of the internal API add some overhead, but the overhead should be the same everywhere so it still allow comparison. This is is more simple to implement and allows comparison with older versions who do not have the same API.