changegroup: add v4 changegroup for revlog v2 exchange This change only adds the required infrastructure for the new changegroup format and does not do any actual exchange. This will be done in the next patches. Differential Revision: https://phab.mercurial-scm.org/D10026

revlogv2: don't assume that the sidedata of the last rev is right after data We are going to be rewriting sidedata soon, it's going to be appended to the revlog data file, meaning that the data and the sidedata might not be contiguous. Differential Revision: https://phab.mercurial-scm.org/D10025