Support git push

[alexanderkjall]

We need to be able to push change up to the git remote.

[mijailr]

I will like to try to solve this.

[alexanderkjall]

My suggestion: This is a pretty large item, might be good to break it down into smaller parts.

First maybe only do a git push to a local master, then do it over ssh, then add some sort of handling of conflicts.

[alexanderkjall]

For future reference, this is how it's done in the crate code: https://github.com/rust-lang/crates.io/blob/57562fdb4760b2351cc8fd89bfca87e06941c978/src/git.rs#L136