revlog: refactor delta chain computation into own function This code is already written in multiple locations. While this code needs to be fast and extracting it to its own function adds overhead, code paths reading delta chains typically read, decompress, and do binary patching on revlog data from the delta chain. This other work (especially zlib decompression) almost certainly accounts for a lot more time than the overhead of introducing a Python function call. So I'm not worried about the performance impact of this change.