commitctx: create the `ChangingFiles` object sooner Let us change the `_commit_manifest` call before other changes.

commitctx: move ChangingFiles creation directly inside `_process_files` As announced, we move it there. We focus on the signature change first, we will update the code afterward.

commitctx: create the ChangingFiles object directly in the various case No need to compute all data then create the object, we can create it early and directly store data in it. We start simple by moving create higher in the function, but the end goal is to eventually move the creation inside the `_process_files` function to take advantage of the object there.

commitctx: no longer make the storage and added/removed file optional The code using this variable is always using other, stricter, condition before using these value. So it is safe to always carry them along.

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.