jujutsu: add ediff option

Based on <https://github.com/martinvonz/jj/wiki/Emacs#ediff-as-a-merge-tool>.

modules/programs/jujutsu.nix

@@ -22,6 +22,15 @@ in {
 
     package = mkPackageOption pkgs "jujutsu" { };
 
+    ediff = mkOption {
+      type = types.bool;
+      default = config.programs.emacs.enable;
+      defaultText = literalExpression "config.programs.emacs.enable";
+      description = ''
+        Enable ediff as a merge tool
+      '';
+    };
+
     settings = mkOption {
       type = tomlFormat.type;
       default = { };
@@ -45,7 +54,18 @@ in {
     home.packages = [ cfg.package ];
 
     home.file.".jjconfig.toml" = mkIf (cfg.settings != { }) {
-      source = tomlFormat.generate "jujutsu-config" cfg.settings;
+      source = tomlFormat.generate "jujutsu-config" (cfg.settings
+        // optionalAttrs (cfg.ediff) (let
+          emacsDiffScript = pkgs.writeShellScriptBin "emacs-ediff" ''
+            set -euxo pipefail
+            ${config.programs.emacs.package}/bin/emacsclient -c --eval "(ediff-merge-files-with-ancestor \"$1\" \"$2\" \"$3\" nil \"$4\")"
+          '';
+        in {
+          merge-tools.ediff = {
+            program = getExe emacsDiffScript;
+            merge-args = [ "$left" "$right" "$base" "$output" ];
+          };
+        }));
     };
   };
 }