rust-cpython: prepare for writing tests that require libpython What I wanted is to disable the "cpython/extension-module<ver>" feature while building tests executable, but that seems not doable. Instead, this patch adds new features dedicated for tests. The make rule is extracted so that we can easily run cargo tests. Added a minimal test of ref-sharing as an example. More tests will follow.

rust-cpython: make inner functions and structs of ref_sharing private Most of these methods were public because they had to be accessible from macro-generated functions. Some "unsafe" can be removed since we can guarantee the data consistency across non-public operations.

rust-cpython: keep Python<'a> token in PyRefMut This just clarifies that the GIL is obtained while PyRefMut is dereferenced, so there's no need of extra acquire_gil() to drop the reference.

rust-cpython: require GIL to borrow immutable reference from PySharedRefCell Since the inner value may be leaked, we probably need GIL to guarantee that there's no data race. inner(py).borrow() is replaced with inner_shared(py).borrow(), which basically means any PySharedRefCell data should be accessed through PySharedRef wrapper.

rust-cpython: make PyLeakedRef operations relatively safe This patch encapsulates the access to the leaked reference to make most leaked-ref operations safe. The only exception is leaked_ref.map(). I couldn't figure out how to allow arbitrary map operation safely over an unsafe static reference. See the docstring and inline comment for details. Now leak_immutable() can be safely implemented as the PyLeakedRef owns its inner data.

rust-cpython: put leaked reference in PyLeakedRef The next patch will make PyLeakedRef manage the lifetime of the underlying object. leak_handle.data.take() will be removed soon.

run-tests: make code coverage work on Python 3 This code path was obviously not tested on Python 3 because it blew up in several places due to str/bytes mismatch. For internal code, we normalize paths to bytes. For code calling into `coverage`, we normalize paths to str, which is what `coverage` seems to expect. After this, `run-tests.py -H` works on Python 3! Differential Revision: https://phab.mercurial-scm.org/D7133

py3: don't index into bytes in phabricator's _tokenize() `phabread`ing a stack using `hg phabread :D1234` under py3 will currently die with a KeyError because it will index into `b':D1234'` and return `58` instead of `b':'` as a token. Differential Revision: https://phab.mercurial-scm.org/D7129

rust-dirstate-status: use fast-path even with fsmonitor and sparse extensions When I initially ran the tests on my series, there were test failures related to those extensions. Now that the initial series has landed, I felt like going back to those issues because people with performance issues will often want to use fsmonitor and sparse. Either because of flaky tests or because the series has changed so much, I can't seem to reproduce these issues... let's widen the scope of the fast-path. Differential Revision: https://phab.mercurial-scm.org/D7128

rust-dirstatemap: remove additional lookups in dirstatemap We're copying this shortcut from the Python implementation, pretty standard for this codebase. Differential Revision: https://phab.mercurial-scm.org/D7117