Add an API endpoint to view the content of a git repo
    
    This will return a JSON blob with the content of a git repo or one of
    its sub-folder or file.
    It will include a link where one can find the actual file or content of
    the folder.
    
    Fixes https://pagure.io/pagure/issue/4808
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>