avoid 31-bit truncation in write_loose_object

The size of the content we are adding may be larger than
2.1G (i.e., "git add gigantic-file"). Most of the code-path
to do so uses size_t or unsigned long to record the size,
but write_loose_object uses a signed int.

On platforms where "int" is 32-bits (which includes x86_64
Linux platforms), we end up passing malloc a negative size.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>