Convoy routing is breadth-first over the fleets actually ordered to convoy
this army, so it returns the shortest chain and answers null rather than
guessing. It takes a set of fleets to treat as already gone, which is what
the adjudicator will need to ask whether a convoy survives being attacked.
The tests found a wrong assumption of mine rather than a wrong line of code:
taking the Channel out of a London to Brest convoy leaves no route at all.
The North Sea does not touch the Mid-Atlantic on this board -- the long way
round is over the top through the Norwegian Sea -- so the Channel is the
whole western sea route.
