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.

copies: return consistent type from revinfo The return is expected to be a list, so we should return a list. This make the use of this function from type-checked language (eg: rust) much simpler.