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>

t/t0064-oid-array.sh

@@ -15,6 +15,24 @@ echoid () {
 	done
 }
 
+test_expect_success 'without repository' '
+	cat >expect <<-EOF &&
+	4444444444444444444444444444444444444444
+	5555555555555555555555555555555555555555
+	8888888888888888888888888888888888888888
+	aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
+	EOF
+	cat >input <<-EOF &&
+	append 4444444444444444444444444444444444444444
+	append 5555555555555555555555555555555555555555
+	append 8888888888888888888888888888888888888888
+	append aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
+	for_each_unique
+	EOF
+	nongit test-tool oid-array <input >actual &&
+	test_cmp expect actual
+'
+
 test_expect_success 'ordered enumeration' '
 	echoid "" 44 55 88 aa >expect &&
 	{