cat-file: report an error on multiple --batch options

The options callback for --batch and --batch-check detects when the two
mutually incompatible options are used. But it simply returns an error
code to parse-options, meaning the program will quit without any kind of
message to the user.

Instead, let's use error() to print something and return -1. Note that
this flips the error return from 1 to -1, but negative values are more
idiomatic here (and parse-options treats them the same).

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

builtin/cat-file.c

@@ -596,7 +596,7 @@ static int batch_option_callback(const struct option *opt,
 	struct batch_options *bo = opt->value;
 
 	if (bo->enabled) {
-		return 1;
+		return error(_("only one batch option may be specified"));
 	}
 
 	bo->enabled = 1;