第三章的AddBlock

开头从数据库取最后一个块的hash是多余的 因为tip已经指向了
`
var lastHash []byte

// 首先获取最后一个块的哈希用于生成新块的哈希
err := bc.db.View(func(tx *bolt.Tx) error {
b := tx.Bucket([]byte(blocksBucket))
lastHash = b.Get([]byte("l"))

return nil
})

if err != nil {
log.Panic(err)
}
`

Start at the Chapter 3 AddBlock entry point and locate the shown database read that retrieves the last block hash from the blocks bucket. Confirm how tip is already used there, then remove the redundant lookup while preserving new-block hash generation.

Written by the indexing model from the issue text.