copies-rust: combine the iteration over remove and copies into one
In the underlying data, the copies information and the removal information are
interleaved. And in the consumer code, the consumption could be interleaved too.
So, we make the processing closer to the underlying data by fusing the two
iterators into one. Later, we will directly consume the underlying data and that
logic to combine the two iterators will be unnecessary.
Differential Revision: https://phab.mercurial-scm.org/D9304
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