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.