fix(retention): keep threads whose post a local user quoted

A quote has a context of its own, so the quoted remote thread counted as
untouched and was evicted, breaking the local quote. On lain.com 45 of
134 quoted posts went this way. The objects_quote_url index keeps the
lookup at one probe per post.

test/mix/tasks/pleroma/database_test.exs

@@ -200,6 +200,29 @@ defmodule Mix.Tasks.Pleroma.DatabaseTest do
       refute Object.get_by_ap_id(old_remote_post2_id)
     end
 
+    test "with the --keep-threads option it keeps old threads a local user quoted", %{
+      old_insert_date: old_insert_date
+    } do
+      remote_user = insert(:user, local: false)
+      local_user = insert(:user)
+
+      {:ok, quoted} = CommonAPI.post(remote_user, %{status: "quote me"})
+
+      quoted
+      |> Ecto.Changeset.change(%{local: false, updated_at: old_insert_date})
+      |> Repo.update!()
+
+      {:ok, quote} = CommonAPI.post(local_user, %{status: "look", quote_id: quoted.id})
+
+      quote
+      |> Ecto.Changeset.change(%{updated_at: old_insert_date})
+      |> Repo.update!()
+
+      Mix.Tasks.Pleroma.Database.run(["prune_objects", "--keep-threads"])
+
+      assert Object.get_by_ap_id(quoted.data["object"])
+    end
+
     test "with the --keep-threads option it keeps old threads addressed to a local user", %{
       old_insert_date: old_insert_date
     } do