perf: add perfrevlogrevision As part of investigating performance improvements to revlog reading, I needed a mechanism to measure every part of revlog reading so I knew where time was spent and how effective optimizations were. This patch implements a perf command for benchmarking the various stages of reading a single revlog revision. When executed against a manifest revision at the end of a 30,000+ long delta chain in mozilla-central, the command demonstrates that ~80% of time is spent in zlib decompression.