ci: verify that an existing sparse checkout can be made unsparse

There are use cases in particular with non-ephemeral runners where an
existing worktree (that has been initialized as a sparse checkout) is
reused in subsequent CI runs (where `actions/checkout` is run _without_
any `sparse-checkout` parameter).

In these scenarios, we need to make sure that the sparse checkout is
disabled before checking out the files.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

.github/workflows/test.yml

@@ -95,6 +95,15 @@ jobs:
       - name: Verify sparse checkout
         run: __test__/verify-sparse-checkout.sh
 
+      # Disabled sparse checkout in existing checkout
+      - name: Disabled sparse checkout
+        uses: ./
+        with:
+          path: sparse-checkout
+
+      - name: Verify disabled sparse checkout
+        run: set -x && test -f src/git-command-manager.ts
+
       # Sparse checkout (non-cone mode)
       - name: Sparse checkout (non-cone mode)
         uses: ./