Create an update-dist-git command
    
    This command is different from 'propose-downstream' in that it works
    offline by default and uses the source-git and dist-git repositories as
    they are: does not switch branches or fetch remotes, it just takes the
    history and content from the source-git repo and creates an update in
    dist-git based on that.
    
    This command is intended to be used in the source-git workflow.
    
    Signed-off-by: Hunor Csomortáni <csomh@redhat.com>