Remove pointless setOldObjectId in test

Setting this value is pointless, because its automatically set
by the refs.newUpdate call that created the update operation.
The API is protected by default, because application level code,
including this test, should not be calling it.

Change-Id: I8867a4e8007892e2bd44a05d7dec619081081943
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

org.eclipse.jgit.test/tst/org/eclipse/jgit/lib/RefUpdateTest.java

@@ -834,7 +834,6 @@ public class RefUpdateTest extends SampleDataRepositoryTestCase {
 			String msg, String refName) throws IOException {
 		RefDirectory refs = (RefDirectory) db.getRefDatabase();
 		RefDirectoryUpdate update = refs.newUpdate(refName, true);
-		update.setOldObjectId(oldId);
 		update.setNewObjectId(newId);
 		refs.log(update, msg, true);
 	}