Add support for content labels on commit

Add commit options which allow for setting labels on commit.
Prevents potential race between garbage collector reading labels
after commit and labels getting set.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

content/local/writer.go

@@ -54,7 +54,7 @@ func (w *writer) Write(p []byte) (n int, err error) {
 	return n, err
 }
 
-func (w *writer) Commit(size int64, expected digest.Digest) error {
+func (w *writer) Commit(size int64, expected digest.Digest, opts ...content.Opt) error {
 	if err := w.fp.Sync(); err != nil {
 		return errors.Wrap(err, "sync failed")
 	}