typing: fix a syntax error in mercurial/cext/bdiff.pyi
I noticed because `merge-pyi` on the non-cext implementations with this file as
input skipped the return type for this. `pytype-single --parse-pyi` confirmed
it was a problem.
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.