patch: support diff data loss detection and upgrade In worst case, generating diff in upgrade mode can be two times more expensive than generating it in git mode directly: we may have to regenerate the whole diff again whenever a git feature is detected. Also, the first diff attempt is completely buffered instead of being streamed. That said, even without having profiled it yet, I am convinced we can fast-path the upgrade mode if necessary were it to be used in regular diff commands, and not only in mq where avoiding data loss is worth the price.