t5000: make hash independent

t5000: make hash independent

This test uses a stub of a very large (64 GB) object to test our
generation of tar archives.  In doing so, it uses the object ID of the
object so it can insert it into the database properly.  Look up these
values using test_oid.  Restructure the test slightly to use
test_oid_in_path.

Since we care about the object, not how it is named in a particular hash
algorithm, rename it to "huge-object", which is shorter and more
descriptive.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>