changegroup: pass function to build delta header into constructor Previously, the delta header struct format was defined on each class and each class had a separate function for building the delta header. We replace both of these with an argument to __init__ containing a callable that can format a delta header given a revisiondelta instance. Differential Revision: https://phab.mercurial-scm.org/D4079

changegroup: make delta header struct formatters actual structs Why we weren't using compiled Struct instances, I don't know. They make code simpler. In theory they are faster. Although I don't believe I was able to measure any meaningful change. That could be because this code is often dominated by compression, deltafication, and function call overhead. Differential Revision: https://phab.mercurial-scm.org/D4078

changegroup: pass version into constructor Currently, the version is an attribute on each class. Passing the argument into the constructor gets us one step closer to eliminating cg2packer and cg3packer. Differential Revision: https://phab.mercurial-scm.org/D4077