Scenario:

• select a line or two
• delete
• press C-z to undo. The lines return and are highlighted.
• press C-y to redo.

Before this commit the editor was crashing:

Error edit.lua:169: assertion failed! Traceback [love "callbacks.lua"]:228: in function 'handler' [C]: in function 'assert' edit.lua:169: in function 'draw' text.lua:1064: in function 'cursor_out_of_screen' edit.lua:487: in function 'keychord_press' run.lua:842: in function 'keychord_press' main.lua:215: in function 'keychord_press' keychord.lua:11: in function <keychord.lua:5> app.lua:31: in function <app.lua:22> [C]: in function 'xpcall'

I was forgetting a step in C-y that was done in C-z. Now undo and redo are symmetric.