This change makes `append_to_pack` completely rely on `p_pwrite` to do
all its I/O instead of splitting it between `p_pwrite` and a
`mmap(2)`/`munmap(2)`+`memcpy(3)`. This saves a good chunk of user CPU
time and avoids making two syscalls per round, but doesn't really cut
down a lot of wall time (~1% on cloning the
[git](https://github.com/git/git.git) repository).