[findBugs] PackWriter.NONE should be final

Change-Id: I4b5621bcb4db82e0560408b3cde6f18b0cc55b29
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/pack/PackWriter.java

@@ -164,7 +164,7 @@ public class PackWriter implements AutoCloseable {
 	private static final int PACK_VERSION_GENERATED = 2;
 
 	/** Empty set of objects for {@code preparePack()}. */
-	public static Set<ObjectId> NONE = Collections.emptySet();
+	public static final Set<ObjectId> NONE = Collections.emptySet();
 
 	private static final Map<WeakReference<PackWriter>, Boolean> instances =
 			new ConcurrentHashMap<WeakReference<PackWriter>, Boolean>();