[Bug] Help text can be navigated and makes closing it with `?` output an error

[adamency]

When printing help text with ? from within Mundo window, we can now navigate to the lines of the help at the top with j and k and mundo will think these are lines of undo nodes, printing en empty diff and making ? (in order to close the help) fail with an error if the cursor is still on the help text.

I believe there should be a mechanism to block navigating to these lines, at least with mundo's own mappings j, k and gg.

Maybe simply store the line number of the top node and limit gg and j to this line.