ci: run codecov job using PR head (#4488)

* ci: run codecov job using PR head

* ci: run all coverage OS on PR, and none on merge group

.github/workflows/ci.yml

@@ -386,40 +386,33 @@ jobs:
       - run: cargo rustdoc --lib --no-default-features --features full -Zunstable-options --config "build.rustdocflags=[\"--cfg\", \"docsrs\"]"
 
   coverage:
+    if : ${{ github.event_name != 'merge_group' }}
     needs: [fmt]
     name: coverage ${{ matrix.os }}
     strategy:
       matrix:
         os: ["windows-latest", "macos-14", "ubuntu-latest"]  # first available arm macos runner
     runs-on: ${{ matrix.os }}
     steps:
-      - if: ${{ github.event_name == 'pull_request' && matrix.os != 'ubuntu-latest' }}
-        id: should-skip
-        shell: bash
-        run: echo 'skip=true' >> $GITHUB_OUTPUT
       - uses: actions/checkout@v4
-        if: steps.should-skip.outputs.skip != 'true'
+        with:
+          # Use the PR head, not the merge commit, because the head commit (and the base)
+          # is what codecov uses to calculate diffs.
+          ref: ${{ github.event.pull_request.head.sha }}
       - uses: actions/setup-python@v5
-        if: steps.should-skip.outputs.skip != 'true'
         with:
           python-version: '3.12'
       - uses: Swatinem/rust-cache@v2
-        if: steps.should-skip.outputs.skip != 'true'
         with:
           save-if: ${{ github.event_name != 'merge_group' }}
       - uses: dtolnay/rust-toolchain@stable
-        if: steps.should-skip.outputs.skip != 'true'
         with:
           components: llvm-tools-preview,rust-src
       - name: Install cargo-llvm-cov
-        if: steps.should-skip.outputs.skip != 'true'
         uses: taiki-e/install-action@cargo-llvm-cov
       - run: python -m pip install --upgrade pip && pip install nox
-        if: steps.should-skip.outputs.skip != 'true'
       - run: nox -s coverage
-        if: steps.should-skip.outputs.skip != 'true'
       - uses: codecov/codecov-action@v4
-        if: steps.should-skip.outputs.skip != 'true'
         with:
           file: coverage.json
           name: ${{ matrix.os }}