perf: clear revlog caches on every iteration
cmdutil.openrevlog() may return a cached revlog instance. This /may/
be a recent "regression" due to refactoring of the manifest API. I'm
not sure.
Either way, this perf command was broken for at least manifests because
subsequent invocations of the perf function would get cache hits from
previous invocations, invalidating results. In the extreme case,
testing the last revision in the revlog resulted in near-instantanous
execution of subsequent runs (since the fulltext is cached). A time
of ~1us would be reported in this case.