Add --no-push option to cherry_picker.py

I've been thinking of adding --no-push option.
It will fetch from upstream, checkout the cherry-pick branch, and do git cherry-pick.
This will allow us to test the change locally before pushing the changes.
Once we're satisfied with it, we can use --continue (implemented in #68) that will do the rest (git push, open web browser, and delete the branch).

What does everyone think about this?