`deepClone` in `fetchgit` does not actually yield a deep clone

A python project I’m building uses setuptools-scm to set the version - this requires the full git history including tags. I tried to use fetchgit with the deepClone = true option, but this still results in a shallow clone. Here’s the derivation:

let
  pkgs = import <nixpkgs> { };
  renku = import (pkgs.fetchgit {
    url = "https://github.com/swissdatasciencecenter/renku-python";
    rev = "refs/heads/nix";
    deepClone = true;
    sha256 = "0hlrnyym2ifivdczmy17r5ipyrs25ay0zr576wc4n283azccbkj5";
  }) { } ;
in with pkgs;
  mkShell {
    pname = "renku";
    version = "0.16.0";
    buildInputs = [ renku ];
    nativeBuildInputs = [ git ];
}

and git log in the cloned directory:

$ git log                                                                                                                                                                                    /nix/store/45c1gh021h74sy8372kswdxyihf5z3r4-renku-python
commit 64afd8a31264e22c511c328f9392b6dc360ce1da (grafted, HEAD -> fetchgit)
Author: Rok Roškar <rok.roskar@example.com>
Date:   Wed Sep 15 02:05:03 2021 +0200

    chore: simplify nix build

Is there some other way that deepClone is supposed to be passed to fetchgit? Or has it been silently deprecated? I know there are many discussions about the downsides of leaveDotGit and deepClone because of unstable .git contents.

Typically we set

SETUPTOOLS_SCM_PRETEND_VERSION = version;

so it will use the version we say it is.

right - but I can’t do that. I need to be able to use the correct version. In addition, fetchgit (because it actually does a shallow clone) makes the repo dirty which causes additional headaches. Is there no way to simply get the equivalent of

git clone <url>
git checkout <ref>