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%.