restore master behavior

up arrow at the top line first goes to char 0, and only goes to the cell
above if already on char 0. Same with down arrow on the bottom line:
transition cursor to the end of the line, and only go down a cell if
already at the end of the last line.

this makes for an unhappy experience in code-mirror's vim mode  for j
and k keys, but we'll fix that in the next commit