Have an interface for the history tree, document it

[lenormf]

Hi,

I edited the title of this issue to reflect the fact that it's not so much about documenting keys as it is about deciding what the interface should be and documenting how the whole undo-tree thing works.

[lenormf]

I had a look at what other editors have, and I think vim's gundo is pretty nice:

• interactive tree visualizer: unlike emacs' implementation, the changes are a stack, not a top-to-bottom list; there's always a left-aligned straight path (representing the current branch) from which the changes branch off as opposed to a centered tree
• diff printing: fancy syntax highlighted diffing to know what the changes do before they're applied

All this combined with tmux panes would be p gr8.

[Delapouite]

I have started a draft here : https://github.com/mawww/kakoune/wiki/Undo-tree to better illustrate the difference between u and <a-u>.

This document deserves a bit of editing and when it will be mature enough, it will be turned into a proper doc/manpages/undo-history.asciidoc.