Get rid of a first N+1 query problem

Use the same approach in Banzai::Filter::MergeRequestReferenceFilter
as we do with issues.
Signed-off-by: Rémy Coutable <remy@rymai.me>

lib/banzai/filter/merge_request_reference_filter.rb

@@ -11,8 +11,8 @@ module Banzai
         MergeRequest
       end
 
-      def find_object(project, id)
-        project.merge_requests.find_by(iid: id)
+      def find_object(project, iid)
+        merge_requests_per_project[project][iid]
       end
 
       def url_for_object(mr, project)
@@ -21,6 +21,25 @@ module Banzai
                                             only_path: context[:only_path])
       end
 
+      # Returns a Hash containing the merge_requests per Project instance.
+      def merge_requests_per_project
+        @merge_requests_per_project ||= begin
+          hash = Hash.new { |h, k| h[k] = {} }
+
+          projects_per_reference.each do |path, project|
+            merge_request_ids = references_per_project[path]
+
+            merge_requests = project.merge_requests.where(iid: merge_request_ids.to_a)
+
+            merge_requests.each do |merge_request|
+              hash[project][merge_request.iid.to_i] = merge_request
+            end
+          end
+
+          hash
+        end
+      end
+
       def object_link_text_extras(object, matches)
         extras = super