pg_rewind: Move syncTargetDirectory() to file_ops.c

pg_rewind: Move syncTargetDirectory() to file_ops.c

For consistency. All the other low-level functions that operate on the
target directory are in file_ops.c.

Reviewed-by: Michael Paquier
Discussion: https://www.postgresql.org/message-id/0c5b3783-af52-3ee5-f8fa-6e794061f70d%40iki.fi