Tweak when we show the merge and the rebase buttons
    
    Only show the merge button when the user is a committer of the project
    against which the PR is opened.
    Show the rebase button when the user is allowed to rebase the PR or has
    commit to the project the PR originates from.
    
    Fixes https://pagure.io/pagure/issue/4596
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>