tests: enable even more Windows server tests

test-obsolete-checkheads: fix on windows

sshserver: avoid a multi-dot attribute lookup in a hot loop This improves stream_out performance by about 3%.

store: reduce string concatenation when joining This improves stream_out performance by a couple of percent.

scmutil: use the new faster path split Combined with a few other patches in this series, this contributes to improving stream_out performance by 10%.

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%.