Restores `jj show` support, broken by the 0.3.0 dispatch rewrite.
`jj show` accepts its revision as a positional `[REVSET]` argument
rather than a `-r` option, which the help-driven dispatch didn't
recognize — so bare `jjf` (which defaults to `show`) errored instead
of opening the picker. A revision named as a bare positional now also
bypasses the picker, like an explicit `-r`.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>