ci: run make sparse as part of the GitHub workflow

Occasionally we receive reviews after patches were integrated, where
`sparse` (https://sparse.docs.kernel.org/en/latest/ has more information
on that project) identified problems such as file-local variables or
functions being declared as global.

By running `sparse` as part of our Continuous Integration, we can catch
such things much earlier. Even better: developers who activated GitHub
Actions on their forks can catch such issues before even sending their
patches to the Git mailing list.

This addresses https://github.com/gitgitgadget/git/issues/345

Note: Not even Ubuntu 20.04 ships with a new enough version of `sparse`
to accommodate Git's needs. The symptom looks like this:

    add-interactive.c:537:51: error: Using plain integer as NULL pointer

To counter that, we download and install the custom-built `sparse`
package from the Azure Pipeline that we specifically created to address
this issue.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

.github/workflows/main.yml

@@ -350,6 +350,27 @@ jobs:
     - uses: actions/checkout@v1
     - run: ci/install-dependencies.sh
     - run: ci/run-static-analysis.sh
+  sparse:
+    needs: ci-config
+    if: needs.ci-config.outputs.enabled == 'yes'
+    env:
+      jobname: sparse
+    runs-on: ubuntu-20.04
+    steps:
+    - name: Download a current `sparse` package
+      # Ubuntu's `sparse` version is too old for us
+      uses: git-for-windows/get-azure-pipelines-artifact@v0
+      with:
+        repository: git/git
+        definitionId: 10
+        artifact: sparse-20.04
+    - name: Install the current `sparse` package
+      run: sudo dpkg -i sparse-20.04/sparse_*.deb
+    - name: Install other dependencies
+      run: |
+        sudo apt-get install -q -y libssl-dev libcurl4-openssl-dev libexpat-dev gettext zlib1g-dev
+    - uses: actions/checkout@v2
+    - run: make sparse
   documentation:
     needs: ci-config
     if: needs.ci-config.outputs.enabled == 'yes'