revlog: switch to a C version of headrevs The C implementation is more than 100 times faster than the Python version (which is still available as a fallback). In a repo with 330,000 revs and a stale .hg/cache/tags file, this patch improves the performance of "hg tip" from 2.2 to 1.6 seconds.

perf: rework perfheads and perftags to clear caches The cache clearing makes numbers more reproducible.