diffhelpers: use Py_ssize_t in testhunk()

Eliminates

  mercurial/diffhelpers.c(143) : warning C4244: '=' : conversion from
  'Py_ssize_t' to 'int', possible loss of data
  mercurial/diffhelpers.c(144) : warning C4244: '=' : conversion from
  'Py_ssize_t' to 'int', possible loss of data

when compiling for Windows x64 target using the Microsoft compiler.

mercurial/diffhelpers.c

@@ -134,8 +134,8 b' testhunk(PyObject *self, PyObject *args)'
 
 	PyObject *a, *b;
 	long bstart;
-	int alen, blen;
-	int i;
+	Py_ssize_t alen, blen;
+	Py_ssize_t i;
 	char *sa, *sb;
 
 	if (!PyArg_ParseTuple(args, "OOl", &a, &b, &bstart))