util: implement a faster os.path.split for posix systems This is not yet used.

scmutil: make join cheaper Combined with a few followup patches, this contributes to improving stream_out performance by 10%.

wireproto: don't format a debug string inside a hot loop This improves stream_out performance by about 5%.

wireproto: bypass filechunkiter for small files when streaming Merely creating and using a generator has a measurable impact, particularly since the common case for stream_out is generators that yield just once. Avoiding generators improves stream_out performance by about 7%.

wireproto: don't audit local paths during stream_out Auditing at this stage is both pointless (paths are already trusted by the local repo) and expensive. Skipping the audits improves stream_out performance by about 15%.

scmutil: delegate mustaudit property to the real opener This will be used by an upcoming patch.

scmutil: turn opener._audit into a property, mustaudit This will be used by an upcoming patch.