toPrettyJson() sorts keys for deterministic output

Summary: Diffing logs would be easier if the lines which pretty-printed JSON were identical if their values were identical, and they're just easier to visually scan for a value if they're sorted anyway.

Reviewed By: yfeldblum, luciang

Differential Revision: D17367669

fbshipit-source-id: 91188f23576ce39b9edc2307be018a43f30c786d

folly/json.cpp

@@ -994,6 +994,7 @@ std::string toJson(dynamic const& dyn) {
 std::string toPrettyJson(dynamic const& dyn) {
   json::serialization_opts opts;
   opts.pretty_formatting = true;
+  opts.sort_keys = true;
   return json::serialize(dyn, opts);
 }