revlog: don't try to partialmatch strings those length > 40

_partialmatch() does prefix matching against nodes. String passed
to _partialmetch() actualy may be any string, not prefix only.

For example,
"e410be8603932e46a51298748a4b874739037fad or 300" is a good
argument for _partialmatch().

When _partialmatch() searches using radix tree, index_partialmatch()
C function shouldn't try to match too long strings.

mercurial/parsers.c

@@ -1084,8 +1084,10 b' static PyObject *index_partialmatch(inde'
 		return NULL;
 	}
 
-	if (nodelen > 40)
-		nodelen = 40;
+	if (nodelen > 40) {
+		PyErr_SetString(PyExc_ValueError, "key too long");
+		return NULL;
+	}
 
 	for (i = 0; i < nodelen; i++)
 		hexdigit(node, i);