bisect: fix handling of `help` and invalid subcommands
As documented in git-bisect(1), `git bisect help` should display usage information. However, since the migration of `git bisect` to a full builtin command in 73fce29427 (Turn `git bisect` into a full built-in, 2022-11-10), this behavior was broken. Running `git bisect help` would, instead of showing usage, either fail silently if already in a bisect session, or otherwise trigger an interactive autostart prompt asking "Do you want me to do it for you [Y/n]?". Similarly, since df63421be9 (bisect--helper: handle states directly, 2022-11-10), running invalid subcommands like `git bisect foobar` also led to the same behavior. This occurred because `help` and other unrecognized subcommands were being unconditionally passed to `bisect_state`, which then called `bisect_autostart`, triggering the interactive prompt. Fix this by: 1. Adding explicit handling for the `help` subcommand to show usage; 2. Validating that unrecognized commands are actually valid state commands before calling `bisect_state`; 3. Showing an error with usage for truly invalid commands. This ensures that `git bisect help` displays the usage as documented, and invalid commands fail cleanly without entering interactive mode. Alternate terms are still handled correctly through `check_and_set_terms`. Signed-off-by: Ruoyu Zhong <zhongruoyu@outlook.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>