Make reorder_dag() maintain list order

[peterbe]

The reorder_dag() function uses a dict to maintain a map of dependencies. This means we lose the primary order of the initial list which might be something we don't want to do.

[peterbe]

Meh. Maybe not. It's hard to control the order very tightly. Dependencies is king.

We might want to consider using an ordereddict anyway just to avoid the randomishness of a dict for maintaining order amongst those jobs with the same dependency stuff.