rust-dirstatemap: implement part of the `setparents` logic The Python code does many round-trip calls to the Rust dirstatemap when copy information needs to be dropped in `setparents`. This may result in improved performance on `commit`, `update` and other such commands, but was mostly done to drop the last use of `set_dirstate_item`. See inline comments for an asterisk about performance, and see next patch for why `set_dirstate_item` has to go. Differential Revision: https://phab.mercurial-scm.org/D12518

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