`fetch-branches` option?

[spenserblack]

My motivation is that some tools (linguist for example) read from git repositories, and so they test against test/* branches in their CI. It would be nice if actions/checkout could also fetch those additional refs.

The workaround is really trivial, and might be so easy that this request is invalid. Just run: git fetch origin extra-branch:extra-branch is all you really need AFAIK. However, I figured it would be nice to have as part this action.