Reduce system overhead "git upload-pack" spends 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
git-compat-util: introduce `cast_size_t_to_ssize_t()`
upload-pack: reduce lock contention when writing packfile data
upload-pack: adapt keepalives based on buffering
upload-pack: fix debug statement when flushing packfile data
2a554ddd35