changegroup: change topics during generation Changegroup generation and consumption currently uses different sets of topics. Generation uses "bundling" and consumption uses the name of the thing being consumed. This commit makes the topic and unit names consistent on both ends. The reason I chose to change the generation side is because when "bundling" is used for 3 different stages, it is unclear which stage of changegroup generation we're in. By making the topics "changesets," "manifests," and "files," one can get a better feel for how far along in the bundling process we are. Differential Revision: https://phab.mercurial-scm.org/D4272