Fix "Out of memory? mmap failed" for files larger than 4GB on Windows

The git_mmap implementation was broken for file sizes that wouldn't fit
into a size_t (32 bits).  This was caused by intermediate variables that
were only 32 bits wide when they should be 64 bits.
Signed-off-by: NJohannes Sixt <j6t@kdbg.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

compat/win32mmap.c

@@ -4,19 +4,19 @@ void *git_mmap(void *start, size_t length, int prot, int flags, int fd, off_t of
 {
 	HANDLE hmap;
 	void *temp;
-	size_t len;
+	off_t len;
 	struct stat st;
 	uint64_t o = offset;
 	uint32_t l = o & 0xFFFFFFFF;
 	uint32_t h = (o >> 32) & 0xFFFFFFFF;
 
 	if (!fstat(fd, &st))
-		len = xsize_t(st.st_size);
+		len = st.st_size;
 	else
 		die("mmap: could not determine filesize");
 
 	if ((length + offset) > len)
-		length = len - offset;
+		length = xsize_t(len - offset);
 
 	if (!(flags & MAP_PRIVATE))
 		die("Invalid usage of mmap when built with USE_WIN32_MMAP");