Fix NPE when notes are present

Change-Id: If9200ae4a7f582a5562aecf323ff0430ba154583
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/RevWalkTextBuiltin.java

@@ -205,7 +205,7 @@ abstract class RevWalkTextBuiltin extends TextBuiltin {
 			return new ObjectWalk(db);
 		if (argWalk != null)
 			return argWalk;
-		return new RevWalk(db);
+		return argWalk = new RevWalk(db);
 	}
 
 	protected int walkLoop() throws Exception {