repartition returns bare Tuple instead of Pair in one branch #542

Every branch of repartition returns dst => U (a Pair) except the _repartition_body(0) identity case, which returns src, U (a Tuple). All current callers destructure so nothing breaks today, but first(repartition(...)) silently means something different for that case.

src/fusiontrees/duality_manipulations.jl:486-490.