(new-)sources should fail with git tracked files
    
    Implements additional protection (except .gitignore warning) that
    prevents overwrite git tracked files when x-pkg sources is run.
    And also forbids uploading tracked files that may not belong
    to the lookaside cache.
    
    Fixes: https://pagure.io/fedpkg/issue/241
    JIRA: COMPOSE-2689
    
    Signed-off-by: Ondrej Nosek <onosek@redhat.com>