core/pkgs/build-support/fetchgitlocal/default.nix

{
  runCommand,
  git,
  lib,
}:

lib.makeOverridable (
  src:

  let
    srcStr = toString src;

    # Adds the current directory (respecting ignored files) to the git store, and returns the hash
    gitHashFile =
      runCommand "put-in-git"
        {
          nativeBuildInputs = [ git ];
          dummy = builtins.currentTime; # impure, do every time
          preferLocalBuild = true;
        }
        ''
          cd ${srcStr}
          DOT_GIT=$(git rev-parse --resolve-git-dir .git) # path to repo

          cp $DOT_GIT/index $DOT_GIT/index-user # backup index
          git reset # reset index
          git add . # add current directory

          # hash of current directory
          # remove trailing newline
          git rev-parse $(git write-tree) \
            | tr -d '\n' > $out

          mv $DOT_GIT/index-user $DOT_GIT/index # restore index
        '';

    gitHash = builtins.readFile gitHashFile; # cache against git hash

    nixPath =
      runCommand "put-in-nix"
        {
          nativeBuildInputs = [ git ];
          preferLocalBuild = true;
        }
        ''
          mkdir $out

          # dump tar of *current directory* at given revision
          git -C ${srcStr} archive --format=tar ${gitHash} \
            | tar xf - -C $out
        '';

  in
  nixPath
)