Turn on many more Eclipse warnings, and fix them

- Warn on empty statements, e.g. "for (;;);". These may be
   typos and are easily replaced by "for (;;) {}" which is more
   explicit.
 - Warn on field hiding. This allows cleanup of many acceptance test
   members, at the cost of a couple of renames and the occasional
   suppression (when the field is in a public nested enum that shadows
   a public constant).
 - Warn on unnecessary casts.
 - Warn on unused declared thrown exceptions. In addition to reducing
   method signature length and number of imports, this also eliminated
   some impossible catch blocks.
 - Warn on missing @Override annotations.
 - Warn on unused parameters. This is likely the most controversial,
   as a few relatively common patterns require unused parameters in a
   way that Eclipse can't ignore. However, it also resulted in cleanup
   of a lot of unnecessary injections and method parameters, so I
   think the cost was worth it.

Change-Id: I7224be8b1c798613a127c88507e8cce400679e5d

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

@@ -30,6 +30,7 @@ import java.util.List;
 
 public class EditDeserializer implements JsonDeserializer<Edit>,
     JsonSerializer<Edit> {
+  @Override
   public Edit deserialize(final JsonElement json, final Type typeOfT,
       final JsonDeserializationContext context) throws JsonParseException {
     if (json.isJsonNull()) {
@@ -73,6 +74,7 @@ public class EditDeserializer implements JsonDeserializer<Edit>,
     return p.getAsInt();
   }
 
+  @Override
   public JsonElement serialize(final Edit src, final Type typeOfSrc,
       final JsonSerializationContext context) {
     if (src == null) {