Add revlog.parentrevs function. This allows one to walk the revision graph using only revision numbers, which can be faster than using revision hashes, especially for RevlogNG, where the parents of a revision are stored as revision numbers.

abort: repository a not found!
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abort: error: Connection refused
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abort: repository a not found!
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abort: destination '../a' already exists
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abort: repository a not found!
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abort: destination 'q' already exists