stack: make a deep copy of `dependencies` before modifying its items The algorithm later on in this method uses .remove() to remove individual elements from items in dependencies, which before this patch modified the cached property contents. So for further use that dictionary was in the form of {1: set([])}, i.e. all sets were empty. This deep copy block could be way simpler, but the problem is that sometimes we get lists of _succs() from evolvebits.builddependencies(). Note: this happens only in topic's stack version of builddependencies() and it looks like a suboptimal way to handle multiple successors (see evolve's counterpart function). stack.builddependencies method is removed, it has served its purpose (see the previous patch).

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