Send source mtime to dist-git
    
    This should be backwards compatible. Dist-git instances should hopefully
    ignore fields they don't understand.
    
    Related: https://github.com/release-engineering/dist-git/pull/22/
    Fixes: https://pagure.io/fedpkg/issue/220
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>