Consolidate use of libgit2 for git operations

[pjbgf]

The image automation controller leverages both libgit2/git2go and go-git for its interactions with git repositories.
Having two git libraries in use means code duplication and complexity that does not add any value to the project.

At present, the main use of go-git is for committing changed manifests:
https://github.com/fluxcd/image-automation-controller/blob/main/controllers/imageupdateautomation_controller.go#L571-L619

Besides that, the use of both libraries is intertwined in most tests.

We should consolidate all operations to a single git library, making it easier to maintain and improve our git interactions.