discovery: fix prepush documentation The prepush documentation claim that when we refuse to push, the second element of the returng tuple is an "outgoing" integer. value should be 0 when no outgoing changeset and 1 otherwise. In pratice if there are no outgoing changeset, "outgoing" value is alway 1 and util.Abort is raised on other error. the stable branch also include this error so it wasn't introduced by a recent refactoring.