copies: split the combination of the copies mapping in its own function In some case, this part take up to 95% of the copy tracing that take about a hundred second. This poor performance comes from the fact we keep duplciating and merging dictionary that are mostly similar. I want to experiment with smarter native code to do this, so I need to isolate the function first.

copies: do not initialize the dictionary with root in changeset copies We don't strictly need the dict to be prefiled with `{}`. Not doing so will make the next changeset simpler. This part of a preparation to use native code for this part of copy tracing.

copies: expand `_chain` variable name to make the function easier to read This came up while explaining what the function is about. I find the function easier to follow that way.

copies: show we can read data from the "first" mapping In practice this make no difference since `t` is a copy of `a`. Having this change before hand will make the next cleanup changeset clearer. This is work on the road to using some native code for some performance critical part of copy tracing.