commands: directly convert ptr to object

When performing an export, directly convert an LFS pointer to an
object using the downloaded object file, skipping over the smudge.

Thanks to @ttaylorr for the implementation of the
NewBlobFromFile() function

commands/command_migrate_export.go

@@ -1,7 +1,6 @@
 package commands
 
 import (
-	"bytes"
 	"fmt"
 	"os"
 	"path/filepath"
@@ -45,15 +44,20 @@ func migrateExportCommand(cmd *cobra.Command, args []string) {
 				return b, nil
 			}
 
-			var buf bytes.Buffer
-
-			if _, err := smudge(gitfilter, &buf, b.Contents, path, false, rewriter.Filter()); err != nil {
+			ptr, err := lfs.DecodePointer(b.Contents)
+			if errors.IsNotAPointerError(err) {
+				return b, nil
+			}
+			if err != nil {
 				return nil, err
 			}
 
-			return &odb.Blob{
-				Contents: &buf, Size: int64(buf.Len()),
-			}, nil
+			downloadPath, err := gitfilter.ObjectPath(ptr.Oid)
+			if err != nil {
+				return nil, err
+			}
+
+			return odb.NewBlobFromFile(downloadPath)
 		},
 
 		TreeCallbackFn: func(path string, t *odb.Tree) (*odb.Tree, error) {

git/odb/blob.go

@@ -3,6 +3,9 @@ package odb
 import (
 	"bytes"
 	"io"
+	"os"
+
+	"github.com/git-lfs/git-lfs/errors"
 )
 
 // Blob represents a Git object of type "blob".
@@ -27,6 +30,42 @@ func NewBlobFromBytes(contents []byte) *Blob {
 	}
 }
 
+// NewBlobFromFile returns a new *Blob that contains the contents of the file
+// at location "path" on disk. NewBlobFromFile does not read the file ahead of
+// time, and instead defers this task until encoding the blob to the object
+// database.
+//
+// If the file cannot be opened or stat(1)-ed, an error will be returned.
+//
+// When the blob receives a function call Close(), the file will also be closed,
+// and any error encountered in doing so will be returned from Close().
+func NewBlobFromFile(path string) (*Blob, error) {
+	f, err := os.Open(path)
+	if err != nil {
+		return nil, errors.Wrapf(err, "git/odb: could not open: %s",
+			path)
+	}
+
+	stat, err := f.Stat()
+	if err != nil {
+		return nil, errors.Wrapf(err, "git/odb: could not stat %s",
+			path)
+	}
+
+	return &Blob{
+		Contents: f,
+		Size:     stat.Size(),
+
+		closeFn: func() error {
+			if err := f.Close(); err != nil {
+				return errors.Wrapf(err,
+					"git/odb: could not close %s", path)
+			}
+			return nil
+		},
+	}, nil
+}
+
 // Type implements Object.ObjectType by returning the correct object type for
 // Blobs, BlobObjectType.
 func (b *Blob) Type() ObjectType { return BlobObjectType }