refs.c: add a strbuf argument to ref_transaction_commit for error logging

Add a strbuf argument to _commit so that we can pass an error string back to
the caller. So that we can do error logging from the caller instead of from
_commit.

Longer term plan is to first convert all callers to use onerr==QUIET_ON_ERR
and craft any log messages from the callers themselves and finally remove the
onerr argument completely.

Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Acked-by: Michael Haggerty <mhagger@alum.mit.edu>

builtin/update-ref.c

@@ -367,7 +367,7 @@ int cmd_update_ref(int argc, const char **argv, const char *prefix)
 		if (end_null)
 			line_termination = '\0';
 		update_refs_stdin();
-		ret = ref_transaction_commit(transaction, msg,
+		ret = ref_transaction_commit(transaction, msg, NULL,
 					     UPDATE_REFS_DIE_ON_ERR);
 		ref_transaction_free(transaction);
 		return ret;