Optimize rs_tube_catchup() a little bit.
Optimize rs_tube_catchup() a little bit.

When the tube is already "caught up" this will short-circuit it's
evaluation better.

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