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
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