How to add applications with their own git history to the advanced structure?


Assume that we’ve got a web application built via Yii2 basic framework. The app’s got its own git history.
Now, we need to add it to a project built upon the advanced boilerplate with its own git history.
I addressed this by utilizing git sub-module. However, it’s made the project very painful to develop.
Is there any better approach?
Can I somehow merge the history of basic app into in the advanced one despite their different directory depth?


Here are what I did. They worked fine.

I deleted git sub-modules by following these instructions:

Then, I merged all three repositories by following this one:

Now, I have the histories of all repositories in a single project.