We have a convention of using -c|-m|FILE elsewhere for reading from
revlogs. Use it for `hg perfrevlog`.
While I was here, I also added a docstring to document what this
command does, as "perfrevlog" is ambiguous.
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.
