tests: stablize test-rename-merge1.t on Windows
This goes with
d7622fdec3b5.
Differential Revision: https://phab.mercurial-scm.org/D8036
rust-cpython: make sure PySharedRef::borrow_mut() never panics
Since it returns a Result, it shouldn't panic depending on where the
borrowing fails.
PySharedRef::borrow_mut() will be renamed to try_borrow_mut() by the next
patch.