This fixes an implicit save when importing files.
The Track's disk-reader is initially created with an empty
source which is later replaced and dropped:
ARDOUR::Session::save_state
ARDOUR::Session::remove_source
PBD::Destructible::drop_references
ARDOUR::DiskWriter::reset_write_sources
ARDOUR::DiskWriter::set_write_source_name
ARDOUR::Track::set_name
Editor::finish_bringing_in_material