Merge branch 'memoize-namespace-root-ancestor' into 'master'

Memoizing root_ancestor in Namespace

See merge request gitlab-org/gitlab-ce!30248

app/models/namespace.rb

@@ -250,7 +250,9 @@ class Namespace < ApplicationRecord
   end
 
   def root_ancestor
-    self_and_ancestors.reorder(nil).find_by(parent_id: nil)
+    strong_memoize(:root_ancestor) do
+      self_and_ancestors.reorder(nil).find_by(parent_id: nil)
+    end
   end
 
   def subgroup?