changegroup: note why a few methods on cg1unpacker exist
I'm not sure what to do abstraction-wise here. It might be more
sensible to make a memoryrepo that could apply a bundle in-memory and
then we could make the changegroup data be strictly an applyable
stream, but that's an idea for Later.
revlog: rename bundle to cg to reflect its nature as a cg?unpacker
The new convention is that bundles contain changegroups. bundle1
happens to *only* be a changegroup, but bundle2 is a more featureful
container that isn't something you can pass to addgroup().