rust-dirstatemap: add `clear_cached_mtime` helper method This will help remove the `get_or_insert` method, which is dangerous because it does not take the `DirstateMap` counters into account. Differential Revision: https://phab.mercurial-scm.org/D12525

rust-dirstatemap: add unit tests These were missing and have already proven valuable since they have found two bugs (fixed in previous patches). There may be other behavior to test, but this gives us a decent coverage. Differential Revision: https://phab.mercurial-scm.org/D12524

rust: add `Debug` trait to a bunch of structs This is useful when... debugging. Right now the output is not in the most readable state it could be, but this is very low effort and is good enough for now. We may want to write a nicer custom debug formatter for some of those structs in the future. Differential Revision: https://phab.mercurial-scm.org/D12523

rust-dirstatemap: use `&HgPath` instead of `HgPathBuf` in `copy_map_insert` No reason to require an owned path here. Differential Revision: https://phab.mercurial-scm.org/D12522

rust-dirstatemap: use `DirstateEntry::tracked` directly `state()` is a legacy API that will be removed at some point, let's use the newer API. Differential Revision: https://phab.mercurial-scm.org/D12521

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

rust-dirstatemap: remove `_drop_entry` This is not used anywhere anymore Differential Revision: https://phab.mercurial-scm.org/D12513

rust-dirstatemap: remove `__settitem__` This is not used anywhere now. Differential Revision: https://phab.mercurial-scm.org/D12512