Merge branch 'jc/push-upstream-sanity'

Fix broken 'push to upstream' implementation.  "git push $there" without
refspec, when the current branch is set to push to a remote different from
$there, used to push to $there using the upstream information to a remote
unreleated to $there.

* jc/push-upstream-sanity:
  push: error out when the "upstream" semantics does not make sense