t/helper: fix segfault in "oid-array" command without repository

t/helper: fix segfault in "oid-array" command without repository

The "oid-array" test helper can supposedly work without a Git
repository, but will in fact crash because `the_repository->hash_algo`
is not initialized. This is because `oid_pos()`, which is used by
`oid_array_lookup()`, depends on `the_hash_algo->rawsz`.

Ideally, we'd adapt `oid_pos()` to not depend on `the_hash_algo`
anymore. That is a bigger untertaking though, so instead we fall back to
SHA1 when there is no repository.

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