Merge branch 'ps/upload-pack-buffer-more-writes'

Reduce system overhead "git upload-pack" spends on relaying "git pack-objects" output to the "git fetch" running on the other end of the connection. * ps/upload-pack-buffer-more-writes: builtin/pack-objects: reduce lock contention when writing packfile data csum-file: drop `hashfd_throughput()` csum-file: introduce `hashfd_ext()` sideband: use writev(3p) to send pktlines wrapper: introduce writev(3p) wrappers compat/posix: introduce writev(3p) wrapper upload-pack: reduce lock contention when writing packfile data upload-pack: prefer flushing data over sending keepalive upload-pack: adapt keepalives based on buffering upload-pack: fix debug statement when flushing packfile data