[github] Fix checkout of PR base commit in pr.yml

Author: kainino0x

.github/workflows/pr.yml

@@ -15,10 +15,14 @@ jobs:
     permissions:
       pull-requests: write
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
         with:
           persist-credentials: false
-      - uses: actions/setup-node@v3
+          # When running a pull_request job, GitHub generates a merge commit that merges the PR
+          # with the latest commit on the target branch. By default, actions/checkout shallowly
+          # checks out only the merge commit. We want to fetch the two parent commits as well.
+          fetch-depth: 2
+      - uses: actions/setup-node@v6
         with:
           node-version: '16.x'
       - run: npm ci
@@ -38,11 +42,10 @@ jobs:
         run: |
           tools/validate --print-case-count-report src/webgpu > case-count-report-after.txt
       - name: checkout before PR
-        env:
-          PR_BASE_REF: ${{ github.event.pull_request.base.ref }}
         run: |
-          git fetch origin "${PR_BASE_REF}"
-          git checkout "${PR_BASE_REF}"
+          # HEAD will always be a merge commit with 2 parents.
+          # HEAD^1 will be the target branch. HEAD^2 will be the PR branch.
+          git checkout HEAD^1
       - name: compute case count before PR and diff
         id: case_count_diff
         run: |