Merge branch 'rj/platform-pread-may-be-thread-unsafe'

On Cygwin, the platform pread(3) is not thread safe, just like our
own compat/ emulation, and cannot be used in the index-pack program.

* rj/platform-pread-may-be-thread-unsafe:
  index-pack: Disable threading on cygwin

diff --cc Makefile
Simple merge

diff --cc builtin/index-pack.c
Simple merge