copies-rust: split up combine_changeset_copies function into a struct
… such that each iteration of its former loop is now a method call,
with the caller driving the loop.
This entirely removes the need for the `DataHolder` hack:
the method now takes a `ChangedFiles<'_>` parameter that borrows
a bytes buffer that can be owned by the caller’s stack frame,
just for the duration of that call.
Differential Revision: https://phab.mercurial-scm.org/D9683
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