better scrolling in diff/log

[extrawurst]

now that the diff and log component know their render height, we can make scrolling/selecting more natural by only scrolling once the selection would leave the view instead of always centering the view over the selection.

[extrawurst]

A little more clarification:
when the selection for would leave the view on the bottom (of log or diff) the view should scroll down one line to keep it in view. similar to the behaviour people know from tools like less and vim.

[extrawurst]

Picture says more than a thousand words, the new behaviour is implemented in log view now as an example: