Add support to retrieve the account diff preferences via REST

The diff preferences of a user can now be retrieved by GET on
'/accounts/*/preferences.diff'.

Change-Id: Ica4014496b8ced4d0d0f34804a8dc4efcdd694a1

gerrit-server/src/main/java/com/google/gerrit/server/account/Module.java

@@ -33,6 +33,7 @@ public class Module extends RestApiModule {
     get(ACCOUNT_KIND, "avatar").to(GetAvatar.class);
     child(ACCOUNT_KIND, "capabilities").to(Capabilities.class);
     get(ACCOUNT_KIND, "groups").to(GetGroups.class);
+    get(ACCOUNT_KIND, "preferences.diff").to(GetDiffPreferences.class);
     get(CAPABILITY_KIND).to(GetCapabilities.CheckOne.class);
   }
 }