difftastic: fix jujutsu integration

Don’t append `--color=always` and `--sort-paths` unconditionally, since
`difftastic` doesn’t like repeated arguments, and the user might have
already set them (via `config.programs.difftastic.options`).

To clarify, if `difft` gets called with any parameter repeated, it will
croak:

    difft --color=always --color=always
    error: the argument '--color <WHEN>' cannot be used multiple times

    Usage: difft [OPTIONS] OLD-PATH NEW-PATH

    For more information, try '--help'.

By patching `cfg.options` we ensure that the parameters are defined, and
have the required value, since the user might have selected differently
for their general setup (again, via `config.programs.difftastic.options`).

modules/programs/difftastic.nix

@@ -188,10 +188,14 @@ in
         programs.jujutsu.settings.ui.diff-formatter = [
           (lib.getExe cfg.package)
         ]
-        ++ (lib.cli.toCommandLineGNU { } cfg.options)
+        ++ (lib.cli.toCommandLineGNU { } (
+          cfg.options
+          // {
+            color = "always";
+            sort-paths = true;
+          }
+        ))
         ++ [
-          "--color=always"
-          "--sort-paths"
           "$left"
           "$right"
         ];