There are two ways to jump to the editor on a specific line: pressing `e` in the
staging or patch building panels, or clicking on a hyperlink in a delta diff. In
both cases, this works perfectly in the unstaged changes view, but in other
views (either staged changes, or an older commit) it can often jump to the wrong
line; this happens when there are further changes to the file being viewed in
later commits or in unstaged changes.
This commit fixes this so that you end up on the right line in these cases.
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).
