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>

__test__/input-helper.test.ts

@@ -79,6 +79,8 @@ describe('input-helper tests', () => {
     expect(settings.clean).toBe(true)
     expect(settings.commit).toBeTruthy()
     expect(settings.commit).toBe('1234567890123456789012345678901234567890')
+    expect(settings.sparseCheckout).toBe(undefined)
+    expect(settings.sparseCheckoutConeMode).toBe(true)
     expect(settings.fetchDepth).toBe(1)
     expect(settings.lfs).toBe(false)
     expect(settings.ref).toBe('refs/heads/some-ref')