Fix bug that caused undo-tree to hang when undoing in region (bug#16377).

Bump copyright year and version number.

Fix bug in menu entries that triggered error on null buffer-undo-tree.

Fix undo/redo-in-region tree diagrams in Commentary.

Install undo-tree undo/redo Edit menu bar items.

Clear undo-tree-visualizer-needs-extending-[up|down] before drawing tree.

Bump version number and copyright year.

Tweak undo history file names to match Emacs backup file names.

Fix bug that cleared tree when enabling undo-tree-mode.

Reinstate accidentally deleted file header.

Update comment about Emacs version required for persistent history.

Fix formatting bug in absolute timestamps.

Don't restore undo history from file when reverting buffer.

Set protected-local property on various visualizer variables.

Remove unused variable.