count-objects: do not use xsize_t when counting object size

count-objects: do not use xsize_t when counting object size

The point of xsize_t is to safely cast an off_t into a size_t
(because we are about to mmap). But in count-objects, we are
summing the sizes in an off_t. Using xsize_t means that
count-objects could fail on a 32-bit system with a 4G
object (not likely, as other parts of git would fail, but
we should at least be correct here).

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>