Make pg_rewind skip files and directories that are removed during server start.

The target cluster that was rewound needs to perform recovery from
the checkpoint created at failover, which leads it to remove or recreate
some files and directories that may have been copied from the source
cluster. So pg_rewind can skip synchronizing such files and directories,
and which reduces the amount of data transferred during a rewind
without changing the usefulness of the operation.

Author: Michael Paquier
Reviewed-by: Anastasia Lubennikova, Stephen Frost and me

Discussion: https://postgr.es/m/20180205071022.GA17337@paquier.xyz

src/backend/replication/basebackup.c

@@ -103,6 +103,9 @@ static TimestampTz throttled_last;
  * The contents of these directories are removed or recreated during server
  * start so they are not included in backups.  The directories themselves are
  * kept and included as empty to preserve access permissions.
+ *
+ * Note: this list should be kept in sync with the filter lists in pg_rewind's
+ * filemap.c.
  */
 static const char *excludeDirContents[] =
 {