C comment: mention why no setting lasterrno in dir_existsfile()

Reported-by: Wei Sun

Discussion: https://postgr.es/m/tencent_1276C08F98579CC19D8A4488C848A8411806@qq.com

Backpatch-through: master

src/bin/pg_basebackup/walmethods.c

@@ -594,6 +594,11 @@ dir_existsfile(WalWriteMethod *wwmethod, const char *pathname)
 
 	fd = open(tmppath, O_RDONLY | PG_BINARY, 0);
 	if (fd < 0)
+
+		/*
+		 * Skip setting dir_data->lasterrno here because we are only checking
+		 * for existence.
+		 */
 		return false;
 	close(fd);
 	return true;