index-pack, unpack-objects: increase input buffer from 4 KiB to 128 KiB

index-pack, unpack-objects: increase input buffer from 4 KiB to 128 KiB

index-pack and unpack-objects both read pack data from stdin through
a 4 KiB static buffer. In index-pack, each fill() flushes consumed
bytes to the pack file via write_or_die(), capping every write(2)
at 4 KiB. unpack-objects uses the same buffer pattern for reads.

On FUSE-backed filesystems every write(2) is a synchronous round
trip through the FUSE protocol (userspace -> kernel -> userspace ->
back), so the 4 KiB buffer turns a clone into many unnecessary tiny
writes with noticeable latency overhead.

Increase the buffer from 4 KiB to 128 KiB. Introduce a shared
DEFAULT_IO_BUFFER_SIZE constant in git-compat-util.h (next to
MAX_IO_SIZE) and use it in index-pack, unpack-objects, and the
hashfile layer in csum-file (which already used 128 KiB but
hardcoded the value).

Syscall counts via strace on HTTPS clones of git/git (~296 MB pack,
5 runs per variant, isolated builds from the same v2.54.0 source):

  index-pack pack file writes: 72,465 -> 24,943 avg (65% fewer)
  total write() syscalls:     310,192 -> 259,530 avg (16% fewer)
  writes of exactly 4096 bytes: ~40,077 -> 0

Wall-clock time of git clone over HTTPS onto a FUSE passthrough
filesystem with writeback caching disabled, 3 runs per variant:

  vscode (~1.26 GB pack): 84.5s -> 75.7s avg (10% faster)
  git/git (~306 MB pack):  22.6s -> 20.0s avg (11% faster)

Signed-off-by: Scott Bauersfeld <sbauersfeld@g.ucla.edu>
Acked-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>