Recent MinGW has a C99 implementation of snprintf functions

Recent MinGW has a C99 implementation of snprintf functions

Starting with MinGW 3.14, released end of 2007, a working snprintf
is available. This means we do not need our own replacement that works
around the broken implementation in Microsoft's C runtime.

People who build git in an old MinGW environment are expected to set
SNPRINTF_RETURNS_BOGUS in their config.mak. msysgit is sufficiently
recent, of course.

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