This broke with 81b497d186 (#3387). In that PR I claimed that we never want to
ask for force-pushing if the server rejected the update, on the assumption that
this can only happen because the remote tracking branch is not up to date, and
users should just fetch in this case. However, I didn't realize it's even
possible to have a branch whose upstream branch is not stored locally; in this
case we can't tell ahead of time whether a force push is going to be necessary,
so we _have_ to rely on the server response to find out. But we only want to do
that in this specific case, so this is not quite an exact revert of 81b497d186.
aac2535104