changegroup: refactor delta parent code We had recently abstracted the delta parent functions to facilitate extracting code from cgpacker. Now that we're in a better place, it is time to revisit the design. Changegroup version 1 requires that the previous node be used as the delta parent. Later versions allow any available node to be used as the base. In the case where an arbitrary parent can be used, the choice of a delta parent is best left in the hands of the storage backend. So it makes sense for the delta parent selection to be hidden away in the storage layer. This means deferring the choice of the delta parent selection function to as close to delta generation time as possible. This commit moves the delta selection logic to essentially just before delta generation. However, because changegroup version 1 limits what we can do, we have retained the ability to force a delta against the previous revision. As part of this, I realized that the ellipsis parent function was unused! That's because ellipsis mode always sends full revisions and not deltas. Differential Revision: https://phab.mercurial-scm.org/D4214

changegroup: differentiate between fulltext and diff based deltas Previously, revisiondelta encoded a delta and an optional prefix containing a delta header. The underlying code could populate the delta with either a real delta or a fulltext revision. Following the theme of wanting to defer serialization of revision data to the changegroup format as long as possible, it seems prudent for the revision delta instance to capture what type of data is being represented. This could possibly allow us to encode revision data differently in the future. But for the short term, it makes the behavior of a revisiondelta more explicit. Differential Revision: https://phab.mercurial-scm.org/D4213

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

changegroup: move manifest chunk emission to generate() 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/D4209

changegroup: move size tracking and end of manifests to generate() Preparing for all the generate* functions to emit data structures instead of raw chunks. Differential Revision: https://phab.mercurial-scm.org/D4208

changegroup: emit delta group close chunk outside of deltagroup() I want to make deltagroup() emit data structures rather than serialized deltas. Upcoming commits will demonstrate why. Differential Revision: https://phab.mercurial-scm.org/D4207

changegroup: extract cgpacker.group() to standalone function It doesn't need to be part of the packer class. Differential Revision: https://phab.mercurial-scm.org/D4206

changegroup: pass all state into group() This will allow us to split it into a standalone function. Differential Revision: https://phab.mercurial-scm.org/D4205