Update .gitignore
    
    Include the typical tarball and expanded tree names, the build logs and the
    arch-named directory where the binary RPMs are saved (for now I included only
    x86_64, I guess including others should be fine too...)
    
    Tested that after `fedpkg local` git status will not complain about the files
    dropped into the git directory.
    
    Signed-off-by: Filipe Brandenburger <filbranden@gmail.com>