index: return False for "len(index) in index"

Since we no longer accept index[len(index)], we should clearly make
"len(index) in index" return False. This should have been part of
a3dacabd476b (index: don't allow index[len(index)] to mean nullid,
2018-07-20)

Differential Revision: https://phab.mercurial-scm.org/D4104

mercurial/cext/revlog.c

@@ -1393,7 +1393,7 b' static int index_contains(indexObject *s'
 
 	if (PyInt_Check(value)) {
 		long rev = PyInt_AS_LONG(value);
-		return rev >= -1 && rev < index_length(self) + 1;
+		return rev >= -1 && rev < index_length(self);
 	}
 
 	if (node_check(value, &node) == -1)