hash-object --stdin: verify that it works with >4GB/LLP64

hash-object --stdin: verify that it works with >4GB/LLP64

Just like the `hash-object --literally` code path, the `--stdin` code
path also needs to use `size_t` instead of `unsigned long` to represent
memory sizes, otherwise it would cause problems on platforms using the
LLP64 data model (such as Windows).

To limit the scope of the test case, the object is explicitly not
written to the object store, nor are any filters applied.

The `big` file from the previous test case is reused to save setup time;
To avoid relying on that side effect, it is generated if it does not
exist (e.g. when running via `sh t1007-*.sh --long --run=1,41`).

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>