config: add CONFIG_ERROR_SILENT handler

We can currently die() or error(), but there's not yet any
way for callers to ask us just to quietly return an error.
Let's give them one.

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

config.c

@@ -818,6 +818,9 @@ static int git_parse_source(config_fn_t fn, void *data,
 	case CONFIG_ERROR_ERROR:
 		error_return = error("%s", error_msg);
 		break;
+	case CONFIG_ERROR_SILENT:
+		error_return = -1;
+		break;
 	case CONFIG_ERROR_UNSET:
 		BUG("config error action unset");
 	}

config.h

@@ -58,6 +58,7 @@ struct config_options {
 		CONFIG_ERROR_UNSET = 0, /* use source-specific default */
 		CONFIG_ERROR_DIE, /* die() on error */
 		CONFIG_ERROR_ERROR, /* error() on error, return -1 */
+		CONFIG_ERROR_SILENT, /* return -1 */
 	} error_action;
 };