merge with stable

rust-cpython: raising error.WdirUnsupported The Graph implementation of hg-cpython returns the appropriate error upon encounter with the working directory special revision number, and this gives us in particular a code path to test from test-rust-ancestors.py In the current implementation, the exception is actually raised from the iterator instantiation; we are nonetheless consuming the iterator in the test with `list()` in order not to depend on implementation details.

rust: error for WdirUnsupported with cpython conversion as exception This introduces WorkingDirectoryUnsupported as an enum variant of GraphError in the core and converts it to the expected `mercurial.error.WdirUnsupported`.