is_native_move() and "Relaxed" move_mode not considering possibility of three road types
(re-raised from gna bug #24242)
cazfi, Dec 2015: is_native_move() allows "Relaxed" diaginal move like through third "between" tile if it has either of the integrating road types that appear in source and destination tile. It does not consider the possibility that it has third road type integrating with both source and destination road types.
cazfi, Feb 2016: More generally: Problem exist when between-tile has integrating road type distinct from both source and destination. This can be third road type, but there's also the case where source and destination are the same and between-tile is the only one with different road type.
(JN: target release of 2.6.0 is as in Gna ticket, but we might want to reconsider. Also, I've got some unpublished half-finished work on integrating road corner cases that may be relevant, although don't wait for me.)