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.
edition = "2018"
max_width = 79
wrap_comments = true
error_on_line_overflow = true