Use default project filter for issue/merge request project searches

Changelog: performance

lib/gitlab/project_search_results.rb

@@ -8,7 +8,8 @@ module Gitlab
       @project = project
       @repository_ref = repository_ref.presence
 
-      super(current_user, query, [project], order_by: order_by, sort: sort, filters: filters)
+      # use the default filter for project searches since we are already limiting by a single project
+      super(current_user, query, [project], order_by: order_by, sort: sort, filters: filters, default_project_filter: true)
     end
 
     def objects(scope, page: nil, per_page: DEFAULT_PER_PAGE, preload_method: nil)

lib/gitlab/search_results.rb

@@ -168,7 +168,7 @@ module Gitlab
       issues = IssuesFinder.new(current_user, issuable_params.merge(finder_params)).execute
 
       unless default_project_filter
-        issues = issues.where(project_id: project_ids_relation) # rubocop: disable CodeReuse/ActiveRecord
+        issues = issues.in_projects(project_ids_relation)
       end
 
       apply_sort(issues, scope: 'issues')