parse-options: make sure argh string does not have SP or _

We encourage to spell an argument hint that consists of multiple
words as a single-token separated with dashes.  In order to help
catching violations added by new callers of parse-options, make sure
argh does not contain SP or _ when the code validates the option
definitions.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

parse-options.c

@@ -375,6 +375,9 @@ static void parse_options_check(const struct option *opts)
 		default:
 			; /* ok. (usually accepts an argument) */
 		}
+		if (opts->argh &&
+		    strcspn(opts->argh, " _") != strlen(opts->argh))
+			err |= optbug(opts, "multi-word argh should use dash to separate words");
 	}
 	if (err)
 		exit(128);