Set protected-local property on various visualizer variables.

Prevents them being cleared by undo-tree-visualizer-mode, so can now set them
before calling it from undo-tree-visualize, making
undo-tree-visualizer-mode-hook more useful. E.g. can add
undo-tree-visualizer-selection-mode to this hook to enable selection-mode by
default.
Don't restore undo history from file when reverting buffer.

revert-buffer explicitly sets buffer-undo-list to nil, so trying to restore
the undo tree from file is arguably the wrong thing to do. Plus this avoids
trying (and failing with a message) to load the undo history when other Emacs
functions revert the buffer (e.g. Magit).