cext: change two more vars to Py_ssize_t in manifest.c
D7913 fixed a compiler warning with a signedness conflict in a ternary operator
by changing the types of some variables to be Py_ssize_t instead of size_t or
int. That commit missed these two cases since they aren't warned about (at least
on my compiler).
Both of these variables are produced by operations on variables that are
themselves Py_ssize_t now/already, so they should keep the same type.
Differential Revision: https://phab.mercurial-scm.org/D7964