parsers: cache the result of index_headrevs Although index_headrevs is much faster than its Python counterpart, it's still somewhat expensive when history is large. Since headrevs is called several times when the tag cache is stale or missing (e.g. after a strip or rebase), there's a win to be gained from caching the result, which we do here.

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.

parsers: reduce raw_length when truncating When stripping revs, we now update raw_length to correctly reflect the new end of the index.

help: inline helper function used once only

help: remove redundant parameter

help: move some helper functions to help.py

help: remove dependency on ui from some helper functions