Reduce risk of deadlocks

We’ve seen a deadlock in CI here https://gitlab.com/mkozono/gitlab-ce/builds/15644492#down-build-trace. This commit should not fix that particular failure, but perhaps it will avoid others.

* Don’t call delete_conflicting_redirects after update if the path wasn’t changed
* Rename descendants without using recursion again, so we can run delete_conflicting_redirects exactly once.