Support reference clones

[sizehnde]

My comment on #1526 sums up the feature request well:

We have implemented an Action internally to support reference clones that essentially has to reimplement the portions of actions/checkout we care about; it would be nice for this to just be a built in feature.

With the introduction of custom runner images, this is even more desireable now, since this type of workflow can be achieved on GitHub-hosted runners now too, not just self-hosted runners.

#1526 appears to be the most recent attempt at implementation, with #1400 being the original potentially?