Optional change in diamond search to continue in the best move
direction until that move turns worse.

This is still WIP since the exact way the new method is to be used is
under investigation. One option is to make it an option in diamond
search and use it only when motion is large.

Overall slightly positive on derfraw300 +0.02%, stdhdraw +0.13%,
but works a lot better for high motion sequences (ex. football : +1%).

Change-Id: If88e01a6021daa0cda934680cdc70be1ee04f798