sparse-checkout: optionally turn off cone mode

While it _is_ true that cone mode is the default nowadays (mainly for
performance reasons: code mode is much faster than non-cone mode), there
_are_ legitimate use cases where non-cone mode is really useful.

Let's add a flag to optionally disable cone mode.

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

README.md

@@ -79,6 +79,10 @@ When Git 2.18 or higher is not in your PATH, falls back to the REST API to downl
     # Default: null
     sparse-checkout: ''
 
+    # Specifies whether to use cone-mode when doing a sparse checkout.
+    # Default: true
+    sparse-checkout-cone-mode: ''
+
     # Number of commits to fetch. 0 indicates all history for all branches and tags.
     # Default: 1
     fetch-depth: ''
@@ -113,6 +117,7 @@ When Git 2.18 or higher is not in your PATH, falls back to the REST API to downl
 
 - [Fetch only the root files](#Fetch-only-the-root-files)
 - [Fetch only the root files and `.github` and `src` folder](#Fetch-only-the-root-files-and-github-and-src-folder)
+- [Fetch only a single file](#Fetch-only-a-single-file)
 - [Fetch all history for all tags and branches](#Fetch-all-history-for-all-tags-and-branches)
 - [Checkout a different branch](#Checkout-a-different-branch)
 - [Checkout HEAD^](#Checkout-HEAD)
@@ -141,6 +146,16 @@ When Git 2.18 or higher is not in your PATH, falls back to the REST API to downl
       src
 ```
 
+## Fetch only a single file
+
+```yaml
+- uses: actions/checkout@v3
+  with:
+    sparse-checkout: |
+      README.md
+    sparse-checkout-cone-mode: false
+```
+
 ## Fetch all history for all tags and branches
 
 ```yaml