source-git init: Raise an error if the dist-git repo is not pristine
    
    As we don't want to commit untracked content to source-git, raise an
    error if the dist-git repo used to initialize the source-git repo is not
    pristine, that is: has untracked files or uncommited changes.
    
    Signed-off-by: Hunor Csomortáni <csomh@redhat.com>