thing.nix

{
  nixpkgs.overlays = [
    (self: super: with super; 
    let
      vimdiff = pkgs.writeScript "vim-diff" "exec ${pkgs.neovim}/bin/nvim -d \"$@\"";
    in
    rec {
      neovim = (super.neovim.override { viAlias = true; vimAlias = true; }).overrideAttrs (oldAttrs: {
        buildCommand = oldAttrs.buildCommand + ''
          ln -s ${vimdiff} $out/bin/vimdiff
        '';
      });
    })
  ];
}