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>

dist/index.js

@@ -1267,7 +1267,8 @@ function getSource(settings) {
             // LFS fetch
             // Explicit lfs-fetch to avoid slow checkout (fetches one lfs object at a time).
             // Explicit lfs fetch will fetch lfs objects in parallel.
-            if (settings.lfs) {
+            // For sparse checkouts, let `checkout` fetch the needed objects lazily.
+            if (settings.lfs && !settings.sparseCheckout) {
                 core.startGroup('Fetching LFS objects');
                 yield git.lfsFetch(checkoutInfo.startPoint || checkoutInfo.ref);
                 core.endGroup();

src/git-source-provider.ts

@@ -188,7 +188,8 @@ export async function getSource(settings: IGitSourceSettings): Promise<void> {
     // LFS fetch
     // Explicit lfs-fetch to avoid slow checkout (fetches one lfs object at a time).
     // Explicit lfs fetch will fetch lfs objects in parallel.
-    if (settings.lfs) {
+    // For sparse checkouts, let `checkout` fetch the needed objects lazily.
+    if (settings.lfs && !settings.sparseCheckout) {
       core.startGroup('Fetching LFS objects')
       await git.lfsFetch(checkoutInfo.startPoint || checkoutInfo.ref)
       core.endGroup()