win32: use _endthreadex to terminate threads, not ExitThread

win32: use _endthreadex to terminate threads, not ExitThread

Because we use the C runtime and
use _beginthreadex to create pthreads,
pthread_exit MUST use _endthreadex.

Otherwise, according to Microsoft:
"Failure to do so results in small
memory leaks when the thread
calls ExitThread."

Simply put, this is not the same as ExitThread.

Signed-off-by: Seija Kijin <doremylover123@gmail.com>
Acked-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>