LocalDiskRefTreeDatabaseTest shall use MockSystemReader

It missed to call the setup() method of its super class which prepares
the MockSystemReader

Change-Id: I39858749f8d0115fc6ac7edc8847ffb2bbc85c33
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/reftree/LocalDiskRefTreeDatabaseTest.java

@@ -80,6 +80,7 @@ public class LocalDiskRefTreeDatabaseTest extends LocalDiskRepositoryTestCase {
 	@Override
 	@Before
 	public void setUp() throws Exception {
+		super.setUp();
 		FileRepository init = createWorkRepository();
 		FileBasedConfig cfg = init.getConfig();
 		cfg.setInt("core", null, "repositoryformatversion", 1);