Add a pthread_cond_timedwait() implementation to the Windows pthread
compatibility layer using SleepConditionVariableCS() with a millisecond
timeout computed from the absolute deadline.
This enables callers to use bounded waits on condition variables
instead of blocking indefinitely with pthread_cond_wait().
Signed-off-by: Paul Tarjan <github@paulisageek.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>