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)