fetchGit: work-around bug in Nix 2.4

Another approach to make `poetry2nix` work with dependencies specified
by arbitrary `rev=<hash>` values on newer Nix revisions.

Relates-to: nix-community#358
Relates-to: NixOS/nix#5128

mk-poetry-dep.nix

@@ -163,11 +163,18 @@ pythonPackages.callPackage
       src =
         if isGit then
           (
-            builtins.fetchGit {
+            builtins.fetchGit ({
               inherit (source) url;
               rev = source.resolved_reference or source.reference;
               ref = sourceSpec.branch or sourceSpec.rev or (if sourceSpec?tag then "refs/tags/${sourceSpec.tag}" else "HEAD");
-            }
+            } // (
+              let
+                nixVersion = builtins.substring 0 3 builtins.nixVersion;
+              in
+              lib.optionalAttrs (builtins.compareVersions nixVersion "2.4" >=  0) {
+                allRefs = true;
+              }
+            ))
           )
         else if isUrl then
           builtins.fetchTarball