Reduce number of gitaly calls in Snippet REST list Endpoints

In this commit, we're removing one Gitaly RPC call by storing
the content of that call and reusing it later.

Changelog: performance

lib/api/entities/snippet.rb

@@ -5,16 +5,22 @@ module API
     class Snippet < BasicSnippet
       expose :author, using: Entities::UserBasic
       expose :file_name do |snippet|
-        snippet.file_name_on_repo || snippet.file_name
+        snippet_files.first || snippet.file_name
       end
       expose :files do |snippet, options|
-        snippet.list_files.map do |file|
+        snippet_files.map do |file|
           {
             path: file,
             raw_url: Gitlab::UrlBuilder.build(snippet, file: file, ref: snippet.repository.root_ref)
           }
         end
       end
+
+      private
+
+      def snippet_files
+        @snippet_files ||= object.list_files
+      end
     end
   end
 end