Hi,

I have 2 questions about GitHub Actions:

1. Could someone please explain to me how actions/checkout@v2 works for pull requests as this seem unclear to me. If my workflow yml looks like this:

on:
  pull_request:
    branches: [master]
...
steps:
  - name: Checkout repository
    uses: actions/checkout@v2
    with:
      ref: ${{ github.event.pull_request.head.sha }}

Any pull request to master branch will checkout the branch that triggered the workflow and run the rest of the workflow (tests) on that branch.
But, what is the default behaviour for PR if I leave it like this (skipping the with: ref: part):

steps:
      - name: Checkout repository
        uses: actions/checkout@v2

Will it checkout and run tests on the branch that triggered the PR as well? If so, is there any reason/adventage to being more specific and adding with: ref:?

2. Is there any particular way of limiting branches that can create a PR to protected branch? Let’s say to allow Dev -> Master, but not random branch -> Master?