parsers: fix width of datalen variable in fm1readmarkers

Because parsers.c does not define PY_SSIZE_T_CLEAN, "s#" format requires
(const char*, int), not (const char*, Py_ssize_t).

https://docs.python.org/2/c-api/arg.html

This error had no problem before 042344313939, where datalen wasn't used.
But now fm1readmarkers() fails with "overflow in obsstore" on Python 2.6.9
(amd64) because upper bits of datalen seem to be filled with 1, making it
a negative integer.

This problem seems not visible on our Python 2.7 environment because upper
bits happen to be filled with 0.

mercurial/parsers.c

@@ -2692,7 +2692,7 b' bail:'
 
 static PyObject *fm1readmarkers(PyObject *self, PyObject *args) {
 	const char *data, *dataend;
-	Py_ssize_t datalen;
+	int datalen;
 	Py_ssize_t offset, stop;
 	PyObject *markers = NULL;