[bug fix] pg_rewind takes long time because it mistakenly copiesdata files

Hello,

Our customer reported that pg_rewind took many hours to synchronize 400GB of data, even if the new primary doesn't
performany updates.  The attached patch fixes that.
 

The cause was that pg_rewind failed to recognize data files in tablespace directories, resulting in the full copy of
thosefiles instead of WAL replay.
 

Regards
Takayuki Tsunakawa