Support undo/redo history across restarts

[pablolagreca]

It would be great to keep undo/redo history after restarts. It has a cost, maybe it can be limited by time and or size of history.

[casouri]

I believe you can archive that with https://melpa.org/#/undo-fu-session

[jdtsmith]

Yes, I use undo-fu-session. One issue with it is that saving undo history tends to build up very long vundo lines of >5-20k, which emacs still doesn't like.

We fixed a bug (see #68) that speeds up draw 10x, but just having such long lines displayed in any buffer slows emacs down perceptibly (including vundo navigation). Long term, it would be nice to draw "only what's needed" in the buffer to avoid this. I'll open an issue.