win32: replace pthread_cond_*() with much simpler code

win32: replace pthread_cond_*() with much simpler code

The Win32 CONDITION_VARIABLE has better performance and is easier to
maintain, as the code is a lot shorter now (the semantics of the
CONDITION_VARIABLE matches the pthread_cond_t very well).

Note: CONDITION_VARIABLE is not available in Windows XP and below,
but the declared minimal Windows version required to build and run
Git for Windows is Windows Vista (which is also beyond its
end-of-life, but for less long than Windows XP), so that's okay.

Signed-off-by: Loo Rong Jie <loorongjie@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>