Materialize LFS objects with an explicit git lfs checkout

Problem

On persistent/self-hosted runners that reuse a working directory across jobs, LFS-tracked files can be left as pointer files instead of real content when a lfs: false checkout precedes a lfs: true checkout of the same repo.

Sequence that triggers it:
 1. A lfs: false run sets GIT_LFS_SKIP_SMUDGE=1, so git checkout writes LFS files to the working tree as pointers.
 2. A later lfs: true run on the same worktree fetches the objects into .git/lfs, but git checkout sees the tree already at the target commit and does not re-smudge the already-present pointer files.

Result: the objects are in the local cache, but the working tree still contains pointers. The action relies on the smudge filter firing during git checkout to materialize content — which holds on a clean tree (GitHub-hosted runners) but not on a reused one.

Related: actions/checkout#270

__test__/git-auth-helper.test.ts

@@ -1065,6 +1065,7 @@ async function setup(testName: string): Promise<void> {
     getWorkingDirectory: jest.fn(() => workspace),
     init: jest.fn(),
     isDetached: jest.fn(),
+    lfsCheckout: jest.fn(),
     lfsFetch: jest.fn(),
     lfsInstall: jest.fn(),
     log1: jest.fn(),

__test__/git-directory-helper.test.ts

@@ -497,6 +497,7 @@ async function setup(testName: string): Promise<void> {
     getWorkingDirectory: jest.fn(() => repositoryPath),
     init: jest.fn(),
     isDetached: jest.fn(),
+    lfsCheckout: jest.fn(),
     lfsFetch: jest.fn(),
     lfsInstall: jest.fn(),
     log1: jest.fn(),

__test__/verify-lfs.sh

@@ -9,3 +9,8 @@ if [ ! -f "./lfs/lfs-file.bin" ]; then
     echo "Expected lfs file does not exist"
     exit 1
 fi
+
+if head -n 1 "./lfs/lfs-file.bin" | grep -q "git-lfs.github.com/spec"; then
+    echo "Expected lfs file to be materialized, but it is still an LFS pointer"
+    exit 1
+fi