Previously, when pressing right-arrow when the cursor is already in the last
hunk, it would jump back to the beginning of that hunk. This can be confusing if
the hunk is long, maybe the start of the hunk is already scrolled off the top of
the window, and then pressing right-arrow actually scrolls *backwards*, which is
counter-intuitive. It's better to do nothing in this case.
Same for left-arrow when the cursor is already in the first hunk, although here
the problem is not so severe (unless diff context was increased by a huge
amount, and the start of the first hunk is scrolled off the bottom of the
window).
Previously, the current line was only moved as much as necessary so that it's in
view again. This had the problem that when jumping downwards from hunk to hunk
with the right-arrow key, only the first line of the new hunk was shown at the
bottom of the window. I prefer to put the selected line in the middle of the
view in this case, so that I can see more of the newly selected hunk.
This has the consequence that when scrolling through the view line by line using
down-arrow, the view jumps by half a screen whenever I reach the bottom. I can
see how some users might be opposed to this change, but I happen to like it too,
because it allows me to see more context of what's ahead.
