// Compare the given data with the stored data (i.e., what Get
// would return). If equal, return nil. If not, return
// CollisionError or DiskHashError (depending on whether the
// data on disk matches the expected hash), or whatever error
// was encountered opening/reading the stored data.
// Compare the given data with the stored data (i.e., what Get
// would return). If equal, return nil. If not, return
// CollisionError or DiskHashError (depending on whether the
// data on disk matches the expected hash), or whatever error
// was encountered opening/reading the stored data.