Open a git repository.
The 'path' argument must point to an existing git repository folder, e.g.
/path/to/my_repo/.git/ (normal repository) objects/ index HEAD
/path/to/bare_repo/ (bare repository) objects/ index HEAD
The method will automatically detect if 'path' is a normal or bare repository or fail is 'path' is neither.
pointer to the repo which will be opened
the path to the repository
0 on success; error code otherwise