xsize_t: check whether we lose bits

Attempting to mmap (via git-add or similar) a file larger than 4GB on
32-bit Linux systems results in a repository that has only the file
modulo 4GB stored, because of truncation of the off_t file size to a
size_t for mmap.

When xsize_t was introduced to handle this truncation in dc49cd7 (Cast
64 bit off_t to 32 bit size_t, 2007-03-06), Shawn even pointed out
that it should detect when such a cutoff happens.

Make it so.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-compat-util.h

@@ -260,6 +260,8 @@ static inline ssize_t xwrite(int fd, const void *buf, size_t len)
 
 static inline size_t xsize_t(off_t len)
 {
+	if (len > (size_t) len)
+		die("Cannot handle files this big");
 	return (size_t)len;
 }