detect current branch
hide intermediate work

previously, going to the previous or next slide
would cause the screen to flash stuff on the screen
for a fraction of a second.
avoid git errors
next slide even if changed

To avoid losing work, pressing the spacebar will usually
not go to the next slide if the file has been modified,
but if that's what you really want to do, you can use
Shift+L and Shift+H.

I wanted to use Shift+Space and Shift+Backspace, but many
terminals do not distinguish between Space and Shift+Space.