copies-rust: use matching to select the final copies information This is a bit more idiomatic and this prepare a future refactoring where InternalCopies from both parent would be updated at the same time. Differential Revision: https://phab.mercurial-scm.org/D9647

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Greg Ward, author of the original bfiles extension

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Na'Tosha Bard of Unity Technologies

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Fog Creek Software

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Special thanks to the University of Toronto and the UCOSP program