Do not push status to feed if its reblog is already inserted (#6488)

A complemental change for precompute_feed_service_spec.rb also fixes its
random failure which is caused by the Snowlake randomization of the order
of an original status and its reblog.

spec/fabricators/list_fabricator.rb

@@ -1,4 +1,4 @@
 Fabricator(:list) do
-  account nil
+  account
   title   "MyString"
 end