parsers: read sizes of metadata pair of obsolete marker at once

This will make it easy to implement bound checking. Currently fm1readmarker()
has no protection for corrupted obsstore and can cause infinite loop or
out-of-bound reads.

mercurial/parsers.c

@@ -2630,12 +2630,12 static PyObject *fm1readmarker(const cha
 	}
 	for (i = 0; i < nmetadata; i++) {
 		PyObject *tmp, *left = NULL, *right = NULL;
-		Py_ssize_t metasize = (unsigned char)(*data++);
-		left = PyString_FromStringAndSize(meta, metasize);
-		meta += metasize;
-		metasize = (unsigned char)(*data++);
-		right = PyString_FromStringAndSize(meta, metasize);
-		meta += metasize;
+		Py_ssize_t leftsize = (unsigned char)(*data++);
+		Py_ssize_t rightsize = (unsigned char)(*data++);
+		left = PyString_FromStringAndSize(meta, leftsize);
+		meta += leftsize;
+		right = PyString_FromStringAndSize(meta, rightsize);
+		meta += rightsize;
 		tmp = PyTuple_New(2);
 		if (!left || !right || !tmp) {
 			Py_XDECREF(left);