revlog: make _partialmatch fail fast on almost-hex inputs Before this change, resolving a revision like [0123456789^] on a large repo can take multiple seconds because: - hg does not realize this is a revset, so it tries various things, including _partialmatch(b"0123456789^") - after the rust lookup fails, it falls back to pure hg - pure hg takes all-but-last chars and converts them to binary, which *succeeds*, so it does the expensive part.