Some options:

- Read a book or tutorials about reimplementing git. That will make clear its data model and how it works.

- Use libgit2 to extract some data from a git repository. As a side effect, you will become a git expert.

- Think what would you like as your source control flow. Use libgit2 or its binding in your favorite language to implement it.

- Adapt git flow or a similar predefined control flow.

- Try Fossil SCM (by the author of SQLite). Not recommended - you will need git in your career.