git-sync: enable automatic package update

pkgs/by-name/gi/git-sync/package.nix

@@ -8,6 +8,7 @@
   gnused,
   makeWrapper,
   inotify-tools,
+  nix-update-script,
 }:
 
 stdenv.mkDerivation rec {
@@ -51,6 +52,10 @@ stdenv.mkDerivation rec {
       --prefix PATH : $wrap_path
   '';
 
+  passthru = {
+    updateScript = nix-update-script { };
+  };
+
   meta = {
     description = "Script to automatically synchronize a git repository";
     homepage = "https://github.com/simonthum/git-sync";