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.

Merge with crew-stable

Merge with stable

mq: preserve --git flag when folding patches Without this, folding a git patch into a regular one downgrades the resulting patch to a regular patch.