mq: qdiff with the same diff options than qrefresh (
issue1350)
Specifically, if current patch is a git patch and mq.git=keep, diff in git
mode.
mq: upgrade to git patch when necessary (
issue767)
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.