Properly reference master branch on central repo (#4713)

As well as the the repo and branch of the PR.

Third time's the charm! [Example
run](https://github.com/datacommonsorg/website/actions/runs/11691607809/job/32559153338?pr=4710)
successfully showing what commits were not in master on a PR opened
against datacommonsorg/website/customdc_stable.

.github/workflows/all-commits-in-master.yml

@@ -10,17 +10,20 @@ jobs:
     steps:
       - uses: actions/checkout@v3
         with:
-          fetch-depth: 0  # Fetch all history for accurate comparison
+          # Fetch all history for accurate comparison
+          fetch-depth: 0
+          # Check out the PR branch
+          ref: ${{ github.event.pull_request.head.ref }}
+          repository: ${{ github.event.pull_request.head.repo.full_name }}
 
       - name: Check if commits exist in master
         run: |
           git remote add dc https://github.com/datacommonsorg/website.git
           git fetch dc
-          MASTER_BRANCH="remotes/dc/master"
-          SOURCE_BRANCH=$(echo ${{ github.head_ref }})
+          MASTER_BRANCH="dc/master"
 
           # Get the list of commits in the source branch that are not in the master branch
-          MISSING_COMMITS=$(git log --pretty="%H - %s" $MASTER_BRANCH..$SOURCE_BRANCH --)
+          MISSING_COMMITS=$(git log --pretty="%H - %s" $MASTER_BRANCH..HEAD --)
 
           if [[ -n "$MISSING_COMMITS" ]]; then
             echo "ERROR: The following commits are not present in $MASTER_BRANCH:"