Bind 'j'/'k' to moving up/down; add next/previous requests bound to Down/Up

This adds support for using these next/previous requests for changing the
current commit in the main view from the diff view, if the diff view has
the main view as it's parent.

Works for both split- and full-screen diff views by making redrawing
conditional when moving (and doing the lowlevel scrolling).

Replaces REQ_MOVE_{UP,DOWN}_ENTER.