The most appropriate context is not always clearly defined. The obvious cases:
For working directory commands, we use None
For commands (eg annotate) with single revs, we use that revision
The less obvious cases:
For commands (eg status, diff) with a pair of revs, we use the second revision
For commands that take a range (like log), we use None
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.
