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

On Cygwin, the platform pread(2) is not thread safe, just like our own
compat/ emulation, and cannot be used in the index-pack program.
Makefile variable NO_THREAD_SAFE_PREAD can be defined to avoid use of
this function in a threaded program.

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