Add support for sparse checkouts (#1369)

* Add support for sparse checkouts

* 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>

* Verify minimum Git version for sparse checkout

The `git sparse-checkout` command is available only since Git version
v2.25.0. The `actions/checkout` Action actually supports older Git
versions than that; As of time of writing, the minimum version is
v2.18.0.

Instead of raising this minimum version even for users who do not
require a sparse checkout, only check for this minimum version
specifically when a sparse checkout was asked for.

Suggested-by: Tingluo Huang <tingluohuang@github.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

* Support sparse checkout/LFS better

Instead of fetching all the LFS objects present in the current revision
in a sparse checkout, whether they are needed inside the sparse cone or
not, let's instead only pull the ones that are actually needed.

To do that, let's avoid running that preemptive `git lfs fetch` call in
case of a sparse checkout.

An alternative that was considered during the development of this patch
(and ultimately rejected) was to use `git lfs pull --include <path>...`,
but it turned out to be too inflexible because it requires exact paths,
not the patterns that are available via the sparse checkout definition,
and that risks running into command-line length limitations.

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

---------

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Co-authored-by: Daniel <daniel.fernandez@feverup.com>

README.md

@@ -74,6 +74,15 @@ When Git 2.18 or higher is not in your PATH, falls back to the REST API to downl
     # Default: true
     clean: ''
 
+    # Do a sparse checkout on given patterns. Each pattern should be separated with
+    # new lines
+    # 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: ''
@@ -106,6 +115,9 @@ When Git 2.18 or higher is not in your PATH, falls back to the REST API to downl
 
 # Scenarios
 
+- [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)
@@ -116,6 +128,34 @@ When Git 2.18 or higher is not in your PATH, falls back to the REST API to downl
 - [Checkout pull request on closed event](#Checkout-pull-request-on-closed-event)
 - [Push a commit using the built-in token](#Push-a-commit-using-the-built-in-token)
 
+## Fetch only the root files
+
+```yaml
+- uses: actions/checkout@v3
+  with:
+    sparse-checkout: .
+```
+
+## Fetch only the root files and `.github` and `src` folder
+
+```yaml
+- uses: actions/checkout@v3
+  with:
+    sparse-checkout: |
+      .github
+      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