Reinstate accidentally deleted file header.
Fix bug that cleared tree when enabling undo-tree-mode.
Tweak undo history file names to match Emacs backup file names.

Fix undo-tree-auto-save-history docstring to make it consistent with what
actually happens.
Bump version number and copyright year.
Clear undo-tree-visualizer-needs-extending-[up|down] before drawing tree.

Fixes bug that could trigger an error when redrawing the tree after toggling
timestamps.
Install undo-tree undo/redo Edit menu bar items.
Fix undo/redo-in-region tree diagrams in Commentary.
Fix bug in menu entries that triggered error on null buffer-undo-tree.
Bump copyright year and version number.