rust-cpython: inline PySharedState::try_borrow_mut() Since the core borrowing/leaking logic has been moved to PySharedRef* and PyLeaked*, it doesn't make sense that PySharedState had a function named "try_borrow_mut". Let's turn it into a pure data struct.

rust-cpython: add panicking version of borrow_mut() and use it The original borrow_mut() is renamed to try_borrow_mut(). Since leak_immutable() no longer incref the borrow count, the caller should know if the underlying value is borrowed or not. No Python world is involved. That's why we can simply use the panicking borrow_mut().

setup: don't skip the search for global hg.exe if there is no local instance The point of trying not to blindly execute `hg` on Windows is that the local hg.exe would be given precedence, and if py3 isn't on PATH, it errors out with a modal dialog. But that's not a problem if there is no local executable that could be run. The problem that I recently ran into was I upgraded the repo format to use zstd. But doing a `make clean` deletes all of the supporting libraries, causing the next run to abort with a message about not understanding the `revlog-compression-zstd` requirement. By getting rid of the local executable in the previous commit when cleaning, we avoid leaving a broken executable around, and avoid the py3 PATH problem too. There is still a small hole in that `hg.exe` needs to be deleted before switching between py2/py3/PyOxidizer builds, because the zstd module won't load. But that seems like good hygiene anyway. Differential Revision: https://phab.mercurial-scm.org/D8038

make: also delete hg.exe when cleaning This will be needed for the next patch, which has more details. It has to come before the call into setup.py because even `python setup.py clean` calls hg to generate the version file. Differential Revision: https://phab.mercurial-scm.org/D8037

merge: start using the per-side copy dicts The point of this patch is mostly to clarify `manifestmerge()`. I find it much easier to reason about now. Differential Revision: https://phab.mercurial-scm.org/D7990

copies: define a type to return from mergecopies() We'll soon return two instances of many of the dicts from `copies.mergecopies()`. That will mean that we need to return 9 different dicts, which is clearly not manageable. This patch instead encapsulates the 4 dicts we'll duplicate in a new type. For now, we still just return one instance of it (plus the separate `diverge` dict). Differential Revision: https://phab.mercurial-scm.org/D7989