dagop: simplify dict/set reuse condition in subsetparentswalker...
dagop: simplify dict/set reuse condition in subsetparentswalker
Prepares for fixing the calculation of p1/p2 sort keys.
With this change, there will be one more copying on merge&fork case. I think
the copying cost is negligible since we'll have to update each item in the
dict on merge/fork.