Look into API Usage optimisations #38

As mentioned here: changesets/action#474

cc @h3rmanj

The API usage when pushing to existing branches could potentially be optimized, if it was easier to emulate a force-push using mutations in the GitHub GraphQL API rather than require multiple API calls to get things into the same state.

There's also a side-effect of preparing the target branch by resetting it first where it will close any existing pull requests from that branch if we're resetting it to the same commit/ref as the base branch of the PR, which is why in https://github.com/changesets/action we have to explicitly re-open the pull request once we've updated the branch.