Set expected old object id to all zeros when head id is null

This will be the case on the initial commit to a repository
and the ref update should expect a non-existent ref

Change-Id: Iaa06e47e6e8cc4a0281c7683b367d4806dd980ea

org.eclipse.jgit/src/org/eclipse/jgit/api/CommitCommand.java

@@ -228,7 +228,10 @@ public class CommitCommand extends GitCommand<RevCommit> {
 							ru.setRefLogMessage(
 									prefix + revCommit.getShortMessage(), false);
 						}
-						ru.setExpectedOldObjectId(headId);
+						if (headId != null)
+							ru.setExpectedOldObjectId(headId);
+						else
+							ru.setExpectedOldObjectId(ObjectId.zeroId());
 						Result rc = ru.forceUpdate();
 						switch (rc) {
 						case NEW: