Group validation fails incorrectly when descendants have permanent redirects

From https://gitlab.com/gitlab-org/gitlab-ce/issues/41786#note_53899101

Steps to reproduce

• Create group 'foo'
• Create subgroup 'foo/bar'
• Rename group 'foo' to 'baz'
• Notice subgroup now has permanent redirect 'foo/bar' => 'baz/bar'
• Try to rename group 'baz' back to 'foo'
• See validation error path 'foo' has been taken before

Expected behavior

• Create group 'foo'
• Create subgroup 'foo/bar'
• Rename group 'foo' to 'baz'
• Notice subgroup now has permanent redirect 'foo/bar' => 'baz/bar'
• Rename group 'baz' back to 'foo'
• Result:
  • 'baz' => 'foo'
  • 'baz/bar' => 'foo/bar'
  • Permanent redirects from 'foo' and 'foo/bar' are destroyed

It seems like we need conflicting_redirects_exists? to return false if the redirect sources are its own descendants.

If we do that, is it possible to break your own Git URLs?

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16397 fixes a specific case, but I think this issue is more general.