Allow using left/right arrow to move side by side diff left/right

MacOS decided to do away with scroll bars unless you're actively scrolling. Unfortunately, this makes left/right scrolling kind of painful (I've forced scroll bars to be on all the time myself, but it is annoying some other users).

The same issue happens to the left/right arrow (not Mac OS specific in this case) when the viewing a new file by using with "j" or "k". For example, when you switch to the next file to review with "j", then the left/right arrow keys stop working but the up and down arrow keys are fine. The focus needs to be set inside diff area so that the left and right arrow keys will work.