Allow setting upstream branch when pushing an "upstreamless" branch

If the local branch does not have an upstream on the remote the push will fail with a similar result:

00:04 Running in /tmp/playground: /usr/bin/git push

fatal: The current branch master2 has no upstream branch.

To push the current branch and set the remote as upstream, use

git push --set-upstream origin master2

The command "/usr/bin/git" terminated with exit code 128.

I think it would be useful if in this case a dialog would be shown for asking if the user would like to create an upstream branch.