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 73fce29 (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 df63421 (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>

Author: ZhongRuoyu

builtin/bisect.c

@@ -1453,9 +1453,13 @@ int cmd_bisect(int argc,
 		if (!argc)
 			usage_msg_opt(_("need a command"), git_bisect_usage, options);
 
+		if (!strcmp(argv[0], "help"))
+			usage_with_options(git_bisect_usage, options);
+
 		set_terms(&terms, "bad", "good");
 		get_terms(&terms);
-		if (check_and_set_terms(&terms, argv[0]))
+		if (check_and_set_terms(&terms, argv[0]) ||
+		    !one_of(argv[0], terms.term_good, terms.term_bad, NULL))
 			usage_msg_optf(_("unknown command: '%s'"), git_bisect_usage,
 				       options, argv[0]);
 		res = bisect_state(&terms, argc, argv);