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
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