修复增量build时实际发生全量build的缺陷

增量build时，会计算源端与目标端的差异，此处没有对目标端文件大小进行赋值(即oldsize)变量，oldsize默认为0，这导致实际发生了全量build

src/bin/pg_rewind/filemap.cpp

@@ -506,6 +506,7 @@ void process_source_file(const char* path, file_type_t type, size_t newsize, con
                  * replayed.
                  */
                 /* mod blocksize 8k to avoid half page write */
+                oldsize = statbuf.oldsize;
                 RewindCompressInfo oldRewindCompressInfo;
                 bool sourceCompressed = info != NULL;
                 bool targetCompressed = isreldatafile && ProcessLocalPca(path, &oldRewindCompressInfo, pg_data);