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.