t6301: make hash size independent

Instead of hard-coding a fixed length example object ID in the test,
compute one using the translation tables.  Move a variable into the
setup block so that we can ensure the exit status of test_oid is
checked.

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

t/t6301-for-each-ref-errors.sh

@@ -5,9 +5,9 @@ test_description='for-each-ref errors for broken refs'
 . ./test-lib.sh
 
 ZEROS=$ZERO_OID
-MISSING=abababababababababababababababababababab
 
 test_expect_success setup '
+	MISSING=$(test_oid deadbeef) &&
 	git commit --allow-empty -m "Initial" &&
 	git tag testtag &&
 	git for-each-ref >full-list &&