Make the pagure hook act as the person doing the push
    
    This was a small annoyance we had, if you were amending a commit from
    someone else, or just rewording the commit message (for example to add
    the ``Merges...`` line) and pushing to the main repo, pagure would show
    the action has being done by the author of the commit rather than the
    person who pushed that commit.
    
    This commit fixes this by relying on the environment variable set by
    gitolite if it is present, else going back to the author of the commit
    if we cannot know who pushed.
    
    Fixes https://pagure.io/pagure/issue/2597
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>