t/oid-info: add empty tree and empty blob values

The testsuite will eventually learn how to run using an algorithm other
than SHA-1. In preparation for this, teach the test_oid family of
functions how to look up the empty blob and empty tree values so they
can be used.

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

t/oid-info/hash-info

@@ -9,3 +9,9 @@ zero sha256:0000000000000000000000000000000000000000000000000000000000000000
 
 algo sha1:sha1
 algo sha256:sha256
+
+empty_blob sha1:e69de29bb2d1d6434b8b29ae775ad8c2e48c5391
+empty_blob sha256:473a0f4c3be8a93681a267e3b1e9a7dcda1185436fe141f7749120a303721813
+
+empty_tree sha1:4b825dc642cb6eb9a060e54bf8d69288fbee4904
+empty_tree sha256:6ef19b41225c5369f1c104d45d8d85efa9b057b53b14b4b9b939dd74decc5321