changegroup: minor cleanups to deltagroup() Differential Revision: https://phab.mercurial-scm.org/D4212

changegroup: emit revisiondelta instances from deltagroup() By abstracting the concept of a delta group away from its serialization (the changegroup format), we make it easier to establish alternate serialization formats. We also make it possible to move aspects of delta group generation into the storage layer. This will allow storage to make decisions about e.g. delta parent choices without the changegroup code needing storage APIs to determine delta parents. We're still a bit of a way from there. Future commits will work towards that world. Differential Revision: https://phab.mercurial-scm.org/D4211

changegroup: move file chunk emission to generate() Same deal as manifests. We want to get to a point where we can emit data structures from deltagroup() and derive the raw changegroup data as late as possible. Differential Revision: https://phab.mercurial-scm.org/D4210