D3453: revlog: use radix tree also for matching keys shorter than 4 hex digits

[Yuya Nishihara]

"I don't know what the reason for the 4-digit limit was,"

I guessed it would avoid building a full radix tree where a given hash
was likely to be ambiguous, but maybe I'm wrong since it seems clear that
linear scan in Python wouldn't be faster than building radix tree in C.