sidedata: enable sidedata computers to optionally rewrite flags Sidedata computers may want to influence the flags of the revision they touch. For example, the computer for changelog-based copytracing can add a flag to signify that this revision might affect copytracing, inversely removing said flag if the information is no longer applicable. See inline documentation in `storageutil` for more details. Differential Revision: https://phab.mercurial-scm.org/D10344

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