changegroup: define functions for creating changegroup packers Currently, we have 3 classes for changegroup generation. Each class handles a specific changegroup format. And each subsequent version's class inherits from the previous one. The interface for the classes is not very well defined and a lot of version-specific behavior is behind overloaded functions. This approach adds complexity and makes changegroup generation difficult to reason about. Upcoming commits will be consolidating these 3 classes so differences between changegroup versions and changegroup generation are controlled by parameters to a single constructor / type rather than by overriding class attributes via inheritance. We begin this process by building dedicated functions for creating each changegroup packer instance. Currently they just call the constructor on the appropriate class. This will soon change. Differential Revision: https://phab.mercurial-scm.org/D4076