cext: fix compiler warning about sign changing line.len is a Py_ssize_t, and we're casing to size_t (unsigned). On my compiler, this causes a warning to be emitted: ``` mercurial/cext/manifest.c: In function 'pathlen': mercurial/cext/manifest.c:48:44: warning: operand of ?: changes signedness from 'Py_ssize_t' {aka 'long int'} to 'long unsigned int' due to unsignedness of other operand [-Wsign-compare] return (end) ? (size_t)(end - l->start) : l->len; ^~~~~~ ``` Differential Revision: https://phab.mercurial-scm.org/D7913