Add core copy detection algorithm
This adds findcopies, which detects merge-relevant copies between
files in a pair of manifests back to the merge ancestor.
While the merge code invokes the copy detection routine, it does not
yet use the result.
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