Fix diffing the branch of a project against the target branch
    
    We used to check the local branches against the local default branch (ie
    most often master) but this failed in a few situation:
    a) when the master of the fork was lagging behind the one of the main
    repo (in which case it would offer a PR for things that had already
    been included)
    b) when the changes in the fork were actually in the master branch
    (since it skipped master to master comparison).
    
    With this commit we are now comparing the local branches to the default
    branch of the main repo, still skipping comparing default to default if
    the repo is not a fork.
    
    Fixes https://pagure.io/pagure/issue/2354
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>