cext: make revlog.c PY_SSIZE_T_CLEAN Without this, Python 3.8 emits a deprecation warning, as using int for # values is deprecated. Many existing modules use PY_SSIZE_T_CLEAN, so this shouldn't be contentious. I audited the file for all # formatters and verified we are using Py_ssize_t everywhere now. Differential Revision: https://phab.mercurial-scm.org/D6196