Merge branch 'id-get-blobs-performance' into 'master'

Memoize old_blob and new_blob to improve performance

See merge request gitlab-org/gitlab-ce!26604

lib/gitlab/diff/file.rb

@@ -133,12 +133,16 @@ module Gitlab
       end
 
       def new_blob
+        strong_memoize(:new_blob) do
           new_blob_lazy&.itself
         end
+      end
 
       def old_blob
+        strong_memoize(:old_blob) do
           old_blob_lazy&.itself
         end
+      end
 
       def new_blob_lines_between(from_line, to_line)
         return [] unless new_blob