Import and call pygit2
    
    Instead of importing some specific function from pygit2 which are then
    called in the code, use the <module>.<function> approach instead.
    
    This will allow us to use more of the pygit2 module content without
    having to change the import line everytime.
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>