Do not trigger RefsChangedEvent on the first attempt to read a ref

Such events make no sense, it has never been visible to this
process so no client can have a stale value of the ref.

Change-Id: Iea3a5035b0a1410b80b09cf53387b22b78b18018
Signed-off-by: Robin Rosenberg <robin.rosenberg@dewire.com>

org.eclipse.jgit/src/org/eclipse/jgit/storage/file/RefDirectory.java

@@ -851,7 +851,7 @@ public class RefDirectory extends RefDatabase {
 	private void fireRefsChanged() {
 		final int last = lastNotifiedModCnt.get();
 		final int curr = modCnt.get();
-		if (last != curr && lastNotifiedModCnt.compareAndSet(last, curr))
+		if (last != curr && lastNotifiedModCnt.compareAndSet(last, curr) && last != 0)
 			parent.fireEvent(new RefsChangedEvent());
 	}