Make tube.c rs_tube_catchup_write() consistent with other code.
Make tube.c rs_tube_catchup_write() consistent with other code.

This makes the structure and trace output of rs_tube_catchup_write() more
consistent with the tube.c copy handling functions.

dbaarda/librsync,dbaarda/librsync,sourcefrog/librsync,librsync/librsync,librsync/librsync,librsync/librsync,dbaarda/librsync,dbaarda/librsync,sourcefrog/librsync,sourcefrog/librsync,sourcefrog/librsync,librsync/librsync