copies: introduce the hg-cpython wrapper for `combine_changeset_copies` This patch focus on the `hg-cpython` part of this work. Bridging the python code with the new rust code in `hg-core`. The next patch will actually plug this in the python code. The rust code use multiple Python callback, python related error within this callback are not expected unless they are a programming error or a data corruption. In addition, these callback will slowly be replaced by native Rust code. For these reasons, we use will deal with unexpected error within this callback using rust Panic and let the `rust-cpython` layer deal with raising a Python exception. The code dealing with the ChangedFile instance is repeating itself a lot. I did not factor these duplication out because that whole code will get replaced by entirely different one in a handful of changesets. Differential Revision: https://phab.mercurial-scm.org/D9298