phases: avoid N² behavior in `advanceboundary` We allowed duplicated entries in the deque, which each entry could potentially insert all its ancestors. So advancing boundary for the full repository would mean each revision would walk all its ancestors, resulting in O(N²) iteration. For repository of any decent size, N² is quickly insane. We introduce a simple set to avoid this and get back to reasonable performance.

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local test passed

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addgroupcopy test passed

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clone test passed

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lowlevelcopy test passed

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slicing test passed

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issnapshot test passed

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findsnapshot test passed