Explicitly disable sparse checkout unless asked for (#1598)

When a worktree is reused by actions/checkout and the first time sparse checkout was enabled, we need to ensure that the second time it is only a sparse checkout if explicitly asked for. Otherwise, we need to disable the sparse checkout so that a full checkout is the outcome of this Action.

## Details
* If no `sparse-checkout` parameter is specified, disable it

This should allow users to reuse existing folders when running
`actions/checkout` where a previous run asked for a sparse checkout but
the current run does not ask for a sparse checkout.

This fixes https://github.com/actions/checkout/issues/1475

There are use cases in particular with non-ephemeral (self-hosted) 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.

### Also includes:

* npm run build
* ci: verify that an existing sparse checkout can be made unsparse
* Added a clarifying comment about test branches.
* `test-proxy` now uses newly-minted `test-ubuntu-git` container image from ghcr.io

---------

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Co-authored-by: John Wesley Walker III <81404201+jww3@users.noreply.github.com>

.github/workflows/test.yml

@@ -7,6 +7,11 @@ on:
       - main
       - releases/*
 
+
+# Note that when you see patterns like "ref: test-data/v2/basic" within this workflow, 
+# these refer to "test-data" branches on this actions/checkout repo.
+# (For example, test-data/v2/basic -> https://github.com/actions/checkout/tree/test-data/v2/basic)
+
 jobs:
   build:
     runs-on: ubuntu-latest
@@ -95,6 +100,16 @@ 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
+        shell: bash
+        run: set -x && ls -l sparse-checkout/src/git-command-manager.ts
+
       # Sparse checkout (non-cone mode)
       - name: Sparse checkout (non-cone mode)
         uses: ./
@@ -175,7 +190,7 @@ jobs:
   test-proxy:
     runs-on: ubuntu-latest
     container:
-      image: alpine/git:latest
+      image: ghcr.io/actions/test-ubuntu-git:main.20240221.114913.703z
       options: --dns 127.0.0.1
     services:
       squid-proxy:
@@ -279,4 +294,4 @@ jobs:
       - name: Fix Checkout v3
         uses: actions/checkout@v3
         with:
-          path: v3
+          path: v3