cg4: introduce protocol flag to signify the presence of sidedata We need a way of signaling whether the current revision has sidedata or not, and re-using the revision flags would waste potential revlog flags and mix two normally independent layers. In this change, we add a single byte at the start of the ch4 delta header to set potential protocol flags. We also reclaim the revlog flag for sidedata, since it is no longer used, in its place now lives the (also experimental) copytracing flag. When generating deltas, apply the `CG_FLAG_SIDEDATA` flag if there is sidedata. When applying the deltas, if said flag is present, the next chunk contains the sidedata. Differential Revision: https://phab.mercurial-scm.org/D10343

changegroup: don't limit cgv4 to revlogv2 repos To help the transition from revlogv1 to revlogv2, we need to be able to enable cgv4 for revlogv1 repos, so that revlogv2 clients can handle adding/removing sidedata over the wire. Differential Revision: https://phab.mercurial-scm.org/D10342

sidedata: gate sidedata functionality to revlogv2 in more places Since revlogv1 is not capable of storing sidedata, we prevent sidedata mechanisms around the revlog layer from doing anything. We however keep the ones that allow a revlogv1 repo to generate sidedata on-the-fly on a push, the pull case simply does not add the sidedata to the revlog. Differential Revision: https://phab.mercurial-scm.org/D10341