Cannot use nixpkgs-review rev HEAD to check local commits added to an existing pull request

[dwt]

This can be seen on NixOS/nixpkgs#366387 where I tried to checkout a PR do some local modifications and then tried to build it. That got me errors like this.

Now... did I do something wrong there or is this expected behavior? Quite possibly I do not understand correctly how it is supposed to work to improve pull requests from other people and make (tested) suggestions on how to improve them.