Eliminate N+1 queries in loading namespaces for every issuable in milestones

If we rely on the helper functions in `GitlabRoutingHelper` for `merge_request_path`,
we end up calling a database query to look up the Namespace association for
every merge request since `entity.project.namespace` is called. By reusing the project
defined in the controller, we avoid that problem.

Partial fix to #27387

app/views/shared/milestones/_issuable.html.haml

@@ -5,7 +5,7 @@
 - base_url_args = [project.namespace.becomes(Namespace), project, issuable_type]
 - can_update = can?(current_user, :"update_#{issuable.to_ability_name}", issuable)
 
-%li{ id: dom_id(issuable, 'sortable'),  class: "issuable-row #{'is-disabled' unless can_update}", 'data-iid' => issuable.iid, 'data-id' => issuable.id, 'data-url' => polymorphic_path(issuable) }
+%li{ id: dom_id(issuable, 'sortable'),  class: "issuable-row #{'is-disabled' unless can_update}", 'data-iid' => issuable.iid, 'data-id' => issuable.id, 'data-url' => polymorphic_path([project.namespace.becomes(Namespace), project, issuable]) }
   %span
     - if show_project_name
       %strong #{project.name} ·

changelogs/unreleased/sh-optimize-milestone-polymorphic-url.yml

+---
+title: Eliminate N+1 queries in loading namespaces for every issuable in milestones
+merge_request:
+author: