How to merge changes from git subtree?

Question

git

Good afternoon. There are two separately started repositories - GUILibs and WorkProject. I decided to add the GUILibs repository as a subtree to the WorkProject. I did everything according to the subtree merging instructions . However, at the stage of the merge, an error occurs

fatal: refusing to merge unrelated histories

I found this article About Git subtree merges . It says that after git 2.9 you should use the flag

--allow-unrelated-histories

Why is there a need to use this flag? Should I always merge with this flag now if I have shared repositories? How to add a subtree correctly? How to make subtree have a common ancestor? It's impossible in my opinion.

Answer 1

in general, to merge a subtree, there is a git subtree command : https://github.com/git/git/tree/master/contrib/subtree
in my opinion, it does everything you need under the hood