when the pager is scrolled to the bottom of the buffer, keep it there.
this should make transcript mode feel a bit more "alive". i've also seen
some confusion about what transcript mode does/doesn't show that i think
has been related to it not pinning scroll.
This fixes a bug where if you ran /diff while at turn was running,
transcript lines would be added to the end of the diff view. Also,
refactor to make this kind of issue less likely in future.
