commitctx: directly gather p1 and p2 copies in `files` The only thing we do with the p1copies and p2copies is to pass them around, we we can gather them later and directly stored them in the `ChangingFiles` object.

commitctx: extract sidedata encoding inside its own function This part of the code is quite independent from the rest. Thank to the new ChangingFiles object, moving with the rest of the sidedata code (in metadata.py) is simple. The changelog.add method is simply passing the `files` object to the new function. It will be easy to increase/change the data we gather and encode without impacting the changelog method.

commitctx: directly pass a ChangingFiles object to changelog.add We pass the rich object to the changelog and it read the field it needs.

commitctx: return a richer object from _prepare_files Instead of returning a lot of different list, we introduce a rich object that hold all the file related information. The unique object help with data consistency and simply functions arguments and return. In the rest of this series we will increase usage of this object to simplify more code.