Add initial MergeOp implementation.

This consists of just the base MergeOp with support for merging LLB
results that include deletions using hardlinks as the efficient path
and copies as fallback.

Signed-off-by: Erik Sipsma <erik@sipsma.dev>

source/http/httpsource_test.go

@@ -9,6 +9,8 @@ import (
 	"testing"
 
 	"github.com/containerd/containerd/content/local"
+	"github.com/containerd/containerd/diff/apply"
+	"github.com/containerd/containerd/diff/walking"
 	ctdmetadata "github.com/containerd/containerd/metadata"
 	"github.com/containerd/containerd/snapshots"
 	"github.com/containerd/containerd/snapshots/native"
@@ -20,6 +22,7 @@ import (
 	"github.com/moby/buildkit/source"
 	"github.com/moby/buildkit/util/leaseutil"
 	"github.com/moby/buildkit/util/testutil/httpserver"
+	"github.com/moby/buildkit/util/winlayers"
 	digest "github.com/opencontainers/go-digest"
 	"github.com/stretchr/testify/require"
 	bolt "go.etcd.io/bbolt"
@@ -359,12 +362,17 @@ func newHTTPSource(tmpdir string) (source.Source, error) {
 		return nil, err
 	}
 	lm := leaseutil.WithNamespace(ctdmetadata.NewLeaseManager(mdb), "buildkit")
+	c := mdb.ContentStore()
+	applier := winlayers.NewFileSystemApplierWithWindows(c, apply.NewFileSystemApplier(c))
+	differ := winlayers.NewWalkingDiffWithWindows(c, walking.NewWalkingDiff(c))
 
 	cm, err := cache.NewManager(cache.ManagerOpt{
 		Snapshotter:    snapshot.FromContainerdSnapshotter("native", containerdsnapshot.NSSnapshotter("buildkit", mdb.Snapshotter("native")), nil),
 		MetadataStore:  md,
 		LeaseManager:   lm,
-		ContentStore:   mdb.ContentStore(),
+		ContentStore:   c,
+		Applier:        applier,
+		Differ:         differ,
 		GarbageCollect: mdb.GarbageCollect,
 	})
 	if err != nil {