This command is similar to Mercurial's revset benchmarking command. It
lets you pass in a file containing revsets. I also included a file
with some revsets to test on the git.git repo. I put it in `testing/`,
which doesn't seem perfect. I'm happy to hear suggestions for better
places, or we can move it later if we find a better place.
Note that these tests don't clear caches between each run (or even
between tests), so revsets that rely on filtering commit data that's
not indexed appear faster than they typically are in reality.
