Clean up various warnings

This patch removes the obsolete imports and updates some of the
deprecated API.

Change-Id: I2c077e786b6acbb0fcbfae053a538757224281a8

gerrit-patch-jgit/src/main/java/org/eclipse/jgit/diff/EditDeserializer.java

@@ -76,7 +76,7 @@ public class EditDeserializer implements JsonDeserializer<Edit>,
   public JsonElement serialize(final Edit src, final Type typeOfSrc,
       final JsonSerializationContext context) {
     if (src == null) {
-      return new JsonNull();
+      return JsonNull.INSTANCE;
     }
     final JsonArray a = new JsonArray();
     add(a, src);