changegroup: make _packmanifests() dumber
The next few patches will rewrite the manifest generation code to work
with merges. We will then walk dirlogs recursively. This prepares for
that by moving much of the treemanifest code out of _packmanifests()
and into generatemanifests(). For this to work, it also adds
_manifestsdone() method that returns the "end of manifests" close
chunk for cg3 and an empty string for cg1 and cg2.