Git release to support different remote and branch

A common case:

A person working on repo X forks it to repo Y and keeps submitting PR's.
He later gets commit access to X. But he prefers to work on the forked
repo Y with his initial workflow of submitting PR's and manually merging
the PR's into X. If he needs to release to git, he now has to clone X
into another directory and run `rake git:release` `because git:release`
only pushes the master branch to origin.

A better workflow would be to create a new remote 'upstream' and create
a local branch 'upstream-branch' which is in sync with the master branch
of upstream. When he wants to cut a release, he can switch to this
branch, update the branch with the commits on upstream and run
`rake release[upstream, upstream-branch]`