ci: run make sparse as a 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 gitgitgadget#345

Note: since we cannot easily replicate the same on Travis (due to the
absence of the GitHub Action `get-azure-pipelines-artifact`), we keep
this separate from the standard CI/PR definition (which exists both as a
GitHub workflow as well as a Travis CI definition).

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

.github/workflows/run-sparse.yml

@@ -0,0 +1,22 @@
+name: Run `sparse`
+
+on: [push, pull_request]
+
+jobs:
+  sparse:
+    runs-on: ubuntu-20.04
+    steps:
+    - name: Download the `sparse` package
+      uses: git-for-windows/get-azure-pipelines-artifact@v0
+      with:
+        repository: git/git
+        definitionId: 10
+        artifact: sparse-20.04
+    - name: Install the `sparse` package
+      run: sudo dpkg -i sparse-20.04/sparse_*.deb
+    - name: Install a couple of 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
+