Fix coverage diffs for PRs that aren't up to date (#2263)

Author: robintown

.github/workflows/test_coverage.yml

@@ -12,6 +12,11 @@ jobs:
     steps:
       - name: Checkout code
         uses: actions/checkout@v2
+        with:
+          # If this is a pull request, make sure we check out its head rather than the
+          # automatically generated merge commit, so that the coverage diff excludes
+          # unrelated changes in the base branch
+          ref: ${{ github.event_name == 'pull_request' && github.event.pull_request.head.sha || '' }}
 
       - name: Yarn cache
         uses: c-hive/gha-yarn-cache@v2