difftastic: migrate to lib.cli.toCommandLineShellGNU

Migrates from the deprecated toCommandLineShell to toCommandLineShellGNU.

This changes the output format to use GNU-style concatenated options
(--color=always) with shell escaping for git config values. Both formats
were verified to work correctly with difftastic.

Signed-off-by: Austin Horstman <khaneliman12@gmail.com>

modules/programs/difftastic.nix

@@ -129,7 +129,7 @@ in
           enable = lib.mkDefault true;
           iniContent =
             let
-              difftCommand = "${lib.getExe cfg.package} ${lib.cli.toGNUCommandLineShell { } cfg.options}";
+              difftCommand = "${lib.getExe cfg.package} ${lib.cli.toCommandLineShellGNU { } cfg.options}";
             in
             mkMerge [
               {