rust-cpython: remove unused API to `drop_entry_and_copy_source` This is not used anywhere anymore and its use cases are covered by the new API Differential Revision: https://phab.mercurial-scm.org/D12520

rust-dirstatemap: remove `set_dirstate_entry`/`set_entry` methods These methods were needed crutches before the Rust implementation caught up to Python. Calling `set_entry` (whether from Python or Rust) was dangerous since it didn't update any of the counters of the DirstateMap data structure, while having no real way of knowing when to use it "correctly" except it you were one of the 3 people who looked deep enough into the soul of this code. Differential Revision: https://phab.mercurial-scm.org/D12519

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

dirstate-item: add missing bit of docstring Differential Revision: https://phab.mercurial-scm.org/D12517

dirstatemap: move `_dirs_incr` and `_dirs_decr` methods out of the common They are only used by the Python implementation now Differential Revision: https://phab.mercurial-scm.org/D12516

dirstatemap: move `_refresh_entry` out of the common methods This is only used in the Python implementation now Differential Revision: https://phab.mercurial-scm.org/D12515

dirstatemap: move `_drop_entry` out of the common methods Only the Python implementation uses it. Differential Revision: https://phab.mercurial-scm.org/D12514