hash: make `is_null_oid()` independent of `the_repository`

hash: make `is_null_oid()` independent of `the_repository`

The function `is_null_oid()` uses `oideq(oid, null_oid())` to check
whether a given object ID is the all-zero object ID. `null_oid()`
implicitly relies on `the_repository` though to return the correct null
object ID.

Get rid of this dependency by always comparing the complete hash array
for being all-zeroes. This is possible due to the refactoring of object
IDs so that their hash arrays are always fully initialized.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>