Fix --dry-run mode of pg_rewind

Even if --dry-run mode was specified, the control file was getting
updated, preventing follow-up runs of pg_rewind to work properly on the
target data folder.  The origin of the problem came from the refactoring
done by ce6afc68.

Author: Alexey Kondratov
Discussion: https://postgr.es/m/7ca88204-3e0b-2f4c-c8af-acadc4b266e5@postgrespro.ru
Backpatch-through: 12

src/bin/pg_rewind/pg_rewind.c

@@ -435,7 +435,8 @@ main(int argc, char **argv)
 	ControlFile_new.minRecoveryPoint = endrec;
 	ControlFile_new.minRecoveryPointTLI = endtli;
 	ControlFile_new.state = DB_IN_ARCHIVE_RECOVERY;
-	update_controlfile(datadir_target, &ControlFile_new, do_sync);
+	if (!dry_run)
+		update_controlfile(datadir_target, &ControlFile_new, do_sync);
 
 	if (showprogress)
 		pg_log_info("syncing target data directory");