[Feature Request] Allow `Push` to work with a branch that has not been synced with upstream , details inside

[adelowo]

So say i just checkout"d master to a new branch (foo) that isn't in the upstream yet, if i try to push .. I would get

fatal: The current branch size_validator has no upstream branch.To push the current branch and set the remote as upstream, use
git push --set-upstream origin size_validator

It would be a nice addition if you could update Push(https://github.com/stevenbraham/gitil/blob/master/commands/sync/sync.go#L22-L24) to support pushing to the foo bar by calling git push --set-upstream origin size_validator if a regular push fails..