mingw: use {gm,local}time_s as backend for {gm,local}time_r

mingw: use {gm,local}time_s as backend for {gm,local}time_r

Since Windows doesn't provide gmtime_r(3) and localtime_r(3),
we're providing a compat version by using non-reentrant gmtime(3) and
localtime(3) as backend. Then, we copy the returned data into the
buffer.

By doing that, in case of failure, we will dereference a NULL pointer
returned by gmtime(3), and localtime(3), and we always return a valid
pointer instead of NULL.

Drop the memcpy(3) by using gmtime_s(), and use localtime_s() as the
backend on Windows, and make sure we will return NULL in case of
failure.

Cc: Johannes Sixt <j6t@kdbg.org>
Cc: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Doan Tran Cong Danh <congdanhqx@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>