Fix pg_rewind to handle relation data files in tablespaces properly.

 Fix pg_rewind to handle relation data files in tablespaces properly.
 
 pg_rewind checks whether each file is a relation data file, from its path.
 Previously this check logic had the bug which made pg_rewind fail to
 recognize any relation data files in tablespaces. Which also caused
 an assertion failure in pg_rewind.
 
 Back-patch to 9.5 where pg_rewind was added.
 
 Author: Takayuki Tsunakawa
 Reviewed-by: Michael Paquier
 Discussion: https://postgr.es/m/0A3221C70F24FB45833433255569204D1F8D6C7A@G01JPEXMBYT05