perf: add asv benchmarks Airspeed velocity (ASV) is a python framework for benchmarking Python packages over their lifetime. The results are displayed in an interactive web frontend. Add ASV benchmarks for mercurial that use contrib/perf.py extension that could be run against multiple reference repositories. The benchmark suite now includes revsets from contrib/base-revsets.txt with variants, perftags, perfstatus, perfmanifest and perfheads. Installation requires asv>=0.2, python-hglib and virtualenv This is part of PerformanceTrackingSuitePlan https://www.mercurial-scm.org/wiki/PerformanceTrackingSuitePlan