generatorset: explicitly track iteration order
The expected iteration order may be different than the fast iteration order (eg:
ancestors(42) is expected to be iterated upward but is fast/lazy to compute
downward.
So we explicitly track the iteration order and enforce it if the manual
iteration is requested.
Default expected iteration order of a generator set is ascending because I'm
not aware of any descending revset that need a generatorset. The first to find
such descending revset will have the pleasure to make this configurable.
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