Problem with implement the equiv of git pull

I am working my way through an implementation of git pull.

It seems that I need to do a Remote.fetch() call followed by a merge.

Problem (1): what is the robust way to generate the fetch_refspecs if the REmote.fetch_refspecs is empty? In other words do you have the APIs that I can duplciaste what push.default=simple causes git pull to do?

Problem (2): I have the objects fetched back I do not know the API to call to merge the remote changed into the local. I tried calling repo.merge() with the ID of the lastest commit from the remote.
But that only lead to modified files in the index. What is the correct API code to make this work?

首先阅读 issue 中提到的 Remote.fetch、Remote.fetch_refspecs 和 Repository.merge API，以及 git pull 如何使用 push.default=simple。确定受支持的 fetch 和 merge 顺序，然后确认从远程获取的更改会合并到本地仓库中，而不是只留下索引修改。