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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