exchangev2: fetch changeset revisions All Mercurial repository data is derived from changesets: you can't do anything unless you have changesets. Therefore, it makes sense for changesets to be the first piece of data that we transfer as part of pull. To do this, we call our new "changesetdata" command, requesting parents and revision data. This gives us all the data that a changegroup delta group would give us. We simply normalize this data into what addgroup() expects and call that API on the changelog to bulk insert revisions into the changelog. Code in this commit is heavily borrowed from changegroup.cg1unpacker.apply(). Differential Revision: https://phab.mercurial-scm.org/D4482