Use setup_work_tree() in builtin-ls-files.c

Signed-off-by: Mike Hommey <mh@glandium.org>
Acked-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-ls-files.c

@@ -525,11 +525,8 @@ int cmd_ls_files(int argc, const char **argv, const char *prefix)
 		break;
 	}
 
-	if (require_work_tree && !is_inside_work_tree()) {
-		const char *work_tree = get_git_work_tree();
-		if (!work_tree || chdir(work_tree))
-			die("This operation must be run in a work tree");
-	}
+	if (require_work_tree && !is_inside_work_tree())
+		setup_work_tree();
 
 	pathspec = get_pathspec(prefix, argv + i);