bdiff: use ssize_t in favor of Py_ssize_t in cpython-unaware locations
This function and struct will be exposed via cffi, so we need to
remove the cpython API dependency they currently have.
#require test-repo
$ . "$TESTDIR/helpers-testrepo.sh"
$ cd "$TESTDIR"/..
New errors are not allowed. Warnings are strongly discouraged.
$ hg files "set:(**.py or **.txt) - tests/**" | sed 's|\\|/|g' |
> python contrib/check-config.py