Brown paper bag fix for MinGW 64-bit stat

When overriding the identifier "stat" so that "struct stat" will be
substituted with "struct _stati64" everywhere, I tried to fix the calls
to the _function_ stat(), too, but I forgot to change the earlier
attempt "stat64" to "_stati64" there.

So, the stat() calls were overridden by calls to _stati64() instead.

Unfortunately, there is a function _stati64() so that I missed that
calls to stat() were not actually overridden by calls to mingw_lstat(),
but t4200-rerere.sh showed the error.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Acked-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/mingw.h

@@ -170,7 +170,7 @@ int mingw_lstat(const char *file_name, struct stat *buf);
 int mingw_fstat(int fd, struct stat *buf);
 #define fstat mingw_fstat
 #define lstat mingw_lstat
-#define stat64(x,y) mingw_lstat(x,y)
+#define _stati64(x,y) mingw_lstat(x,y)
 
 int mingw_utime(const char *file_name, const struct utimbuf *times);
 #define utime mingw_utime