git: option to use difftastic as difftool (#5335)

I want `git diff` to stay the same, but `git difftool` to use
difftastic, as described in the difftastic docs:
https://difftastic.wilfred.me.uk/git.html#regular-usage

modules/programs/git.nix

@@ -283,6 +283,13 @@ in {
 
         package = mkPackageOption pkgs "difftastic" { };
 
+        enableAsDifftool = mkEnableOption "" // {
+          description = ''
+            Enable the {command}`difftastic` syntax highlighter as a git difftool.
+            See <https://github.com/Wilfred/difftastic>.
+          '';
+        };
+
         background = mkOption {
           type = types.enum [ "light" "dark" ];
           default = "light";
@@ -656,18 +663,28 @@ in {
       };
     })
 
-    (mkIf cfg.difftastic.enable {
+    (let
-      home.packages = [ cfg.difftastic.package ];
+      difftCommand = concatStringsSep " " [
-
+        "${getExe cfg.difftastic.package}"
-      programs.git.iniContent = let
+        "--color ${cfg.difftastic.color}"
-        difftCommand = concatStringsSep " " [
+        "--background ${cfg.difftastic.background}"
-          "${getExe cfg.difftastic.package}"
+        "--display ${cfg.difftastic.display}"
-          "--color ${cfg.difftastic.color}"
+      ];
-          "--background ${cfg.difftastic.background}"
+    in (lib.mkMerge [
-          "--display ${cfg.difftastic.display}"
+      (mkIf cfg.difftastic.enable {
-        ];
+        home.packages = [ cfg.difftastic.package ];
-      in { diff.external = difftCommand; };
+        programs.git.iniContent = { diff.external = difftCommand; };
-    })
+      })
+      (mkIf cfg.difftastic.enableAsDifftool {
+        home.packages = [ cfg.difftastic.package ];
+        programs.git.iniContent = {
+          diff = { tool = lib.mkDefault "difftastic"; };
+          difftool = {
+            difftastic = { cmd = "${difftCommand} $LOCAL $REMOTE"; };
+          };
+        };
+      })
+    ]))
 
     (let
       deltaPackage = cfg.delta.package;