Merge branch 'jc/push-upstream-sanity' into maint

The 'push to upstream' implementation was broken in some corner cases. "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