i18n: factorize "invalid value" messages

Use the same message when an invalid value is passed to a command line
option or a configuration variable.

Signed-off-by: Jean-Noël Avila <jn.avila@free.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

setup.c

@@ -559,7 +559,8 @@ static enum extension_result handle_extension(const char *var,
 			return config_error_nonbool(var);
 		format = hash_algo_by_name(value);
 		if (format == GIT_HASH_UNKNOWN)
-			return error("invalid value for 'extensions.objectformat'");
+			return error(_("invalid value for '%s': '%s'"),
+				     "extensions.objectformat", value);
 		data->hash_algo = format;
 		return EXTENSION_OK;
 	}