hash-object: add another >4GB/LLP64 test case

hash-object: add another >4GB/LLP64 test case

To complement the `--stdin` and `--literally` test cases that verify
that we can hash files larger than 4GB on 64-bit platforms using the
LLP64 data model, here is a test case that exercises `hash-object`
_without_ any options.

Just as before, we use the `big` file from the previous test case if it
exists to save on setup time, otherwise generate it.

Signed-off-by: Philip Oakley <philipoakley@iee.email>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>