alternative route has detour for turn_costs=true

[karussell]

For this route there is a strange detour circle in the middle which disappears for turn_costs=false.

[michaz]

Ahhh, right. We need to concatenate "edge-based paths" properly.

Suppose we have a non-shortest candidate path s----u-v-w----t. Now we want a concatenation of two shortest paths via v. In node-based search, we take concat(s->v, v->t). In edge-based, we were taking concat(s->u-v, v-w->t). But that can be an ugly path (see above), because maybe from v-w we need to get back via u-v again.

Instead, we want concat(s->u-v, u-v->t). Concatenating "by edge", of course. (Removing the duplicate u-v edge.)