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)
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