Do not use date.c:tm_to_time_t() from compat/mingw.c

Do not use date.c:tm_to_time_t() from compat/mingw.c

To implement gettimeofday(), a broken-down UTC time was requested from the
system using GetSystemTime(), then tm_to_time_t() was used to convert it
to a time_t because it does not look at the current timezone, which
mktime() would do.

Use GetSystemTimeAsFileTime() and a different conversion path to avoid this
back-reference from the compatibility layer to the generic code.

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