cache matched defaults sets for given parameters (#6888)

Merge pull request 6888

lib/jekyll/frontmatter_defaults.rb

@@ -188,10 +188,14 @@ module Jekyll
     #
     # Returns an array of hashes
     def matching_sets(path, type)
+      @matched_set_cache ||= {}
+      @matched_set_cache[path] ||= {}
+      @matched_set_cache[path][type] ||= begin
         valid_sets.select do |set|
           !set.key?("scope") || applies?(set["scope"], path, type)
         end
       end
+    end
 
     # Returns a list of valid sets
     #