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.

local test passed
addgroupcopy test passed
clone test passed
lowlevelcopy test passed
slicing test passed
issnapshot test passed
findsnapshot test passed