xsize_t: check whether we lose bits

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>