perf: update perfindex to be more realistic
The previous code was creating a revlog manually, we now use the actual
`localrepo` method to create it.
We have to jump though extra hops to work around the impact of filecache.
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.