checkout-index: disallow "--no-stage" option

We do not really expect people to use "--no-stage", but if
they do, git currently segfaults. We could instead have it
undo the effects of a previous "--stage", but this gets
tricky around the "to_tempfile" flag. We cannot simply reset
it to 0, because we don't know if it was set by a previous
"--stage=all" or an explicit "--temp" option.

We could solve this by setting a flag and resolving
to_tempfile later, but it's not worth the effort. Nobody
actually wants to use "--no-stage"; we are just trying to
fix a potential segfault here.

While we're in the area, let's improve the user-facing
messages for this option. The error string should be
translatable, and we should give some hint in the "-h"
output about what can go in the argument field.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/checkout-index.c

@@ -141,7 +141,7 @@ static int option_parse_stage(const struct option *opt,
 		if ('1' <= ch && ch <= '3')
 			checkout_stage = arg[0] - '0';
 		else
-			die("stage should be between 1 and 3 or all");
+			die(_("stage should be between 1 and 3 or all"));
 	}
 	return 0;
 }
@@ -173,9 +173,9 @@ int cmd_checkout_index(int argc, const char **argv, const char *prefix)
 			N_("write the content to temporary files")),
 		OPT_STRING(0, "prefix", &state.base_dir, N_("string"),
 			N_("when creating files, prepend <string>")),
-		OPT_CALLBACK(0, "stage", NULL, NULL,
+		{ OPTION_CALLBACK, 0, "stage", NULL, "1-3|all",
 			N_("copy out the files from named stage"),
-			option_parse_stage),
+			PARSE_OPT_NONEG, option_parse_stage },
 		OPT_END()
 	};