Use github.head_ref as branch default value

[stefanzweifel]

${{ github.head_ref }} is only filled when a workflow listens to the pull_request event. However, it evaluates to an empty string when listening to other events (the same default value as the current version has).

By doing this change, the branch-input no longer must manually added to a workflow when listening to pull_request. But, the actions/checkout step still has to be updated and ref has to be added. Otherwise, the repo would be cloned in a detached head and this Action can't commit and push correctly

    - uses: actions/checkout@v2
      with:
        ref: ${{ github.head_ref }}

refs #73

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I'm not sure yet, if this is the right approach.
Actions are basically just configuration code. I think being more explicit in that configuration code leads to less confusion and easier to understand workflows.

In addition, the user still needs to update the checkout-step. So a consumer can't just add 2-3 lines of code to add one single step. They have to update the rest of their workflow too. 🤔

[spawnia]

I can not think of a scenario where the default '' would be better.

Sane defaults generally make things easier to use and are also a great way to smooth over potential upgrades or breakages. It saves every user the trouble of considering possible configuration values.

[stefanzweifel]

Yeah, I think this is the way to go.