Add movement shortcut for history

[pickfire]

alt-u and alt-U

[pickfire]

cc @Omnikar since he worked on #981

[pickfire]

I wonder would if be useful if we mentioned to the user how many steps were done?

[Omnikar]

I haven't really looked at the code but… I'm confused; what's the difference between this and undo/redo?

[pickfire]

insert a

a

insert a

aa

insert a

aaa

undo u

aa

insert b

aab

move backward in history altu

aaa

move forward in history altU

aab

Basically if you messed up after you undo, you can still go back. Like undo tree but think it is two dimension, the normal undo is one branch of the undo tree, but this traverse by time of the undo tree rather than by branch.

Previously we have :earlier and :later for this which is more powerful but it is good to have a key for this. Vim have this as g- and g+ and kakoune have the same keys used here.

[Omnikar]

Ah, that's quite useful.