Merge branch 'js/difftool-no-index'

The "--dir-diff" mode of "git difftool" is not useful in "--no-index"
mode; they are now explicitly marked as mutually incompatible.

* js/difftool-no-index:
  difftool --no-index: error out on --dir-diff (and don't crash)

builtin/difftool.c

@@ -727,7 +727,8 @@ int cmd_difftool(int argc, const char **argv, const char *prefix)
 		setup_work_tree();
 		setenv(GIT_DIR_ENVIRONMENT, absolute_path(get_git_dir()), 1);
 		setenv(GIT_WORK_TREE_ENVIRONMENT, absolute_path(get_git_work_tree()), 1);
-	}
+	} else if (dir_diff)
+		die(_("--dir-diff is incompatible with --no-index"));
 
 	if (use_gui_tool + !!difftool_cmd + !!extcmd > 1)
 		die(_("--gui, --tool and --extcmd are mutually exclusive"));