Package undo-tree provided by this package replaces Emacs' undo
 system with a system that treats undo history as what it is: a
 branching tree of changes. This simple idea allows the more intuitive
 behaviour of the standard undo/redo system to be combined with the
 power of never losing any history. An added side bonus is that undo
 history can in some cases be stored more efficiently, allowing more
 changes to accumulate before Emacs starts discarding history.