copies-rust: move CPU-heavy Rust processing into a child thread … that runs in parallel with the parent thread fetching data. This can be disabled through a new config. CLI example: hg --config=devel.copy-tracing.multi-thread=no For now both threads use the GIL, later commits will reduce this. Differential Revision: https://phab.mercurial-scm.org/D9684

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

copies-rust: extract generic map merge logic from merge_copies_dict This deduplicates the copy-tracing-specific logic Differential Revision: https://phab.mercurial-scm.org/D9682