t/test-lib: introduce ZERO_OID

t/test-lib: introduce ZERO_OID

Currently we have a variable, $_z40, which contains the all-zero object
ID.  However, with NewHash, we'll have an all-zero object ID which is
longer than 40 hex characters.  In such a case, $_z40 will be a
confusing name.  Create a $ZERO_OID variable which will always reflect
the all-zeros object ID, regardless of the length of the current hash.

Signed-off-by: brian m. carlson <redacted>
Signed-off-by: Junio C Hamano <redacted>