D6561: copies: simplify merging of copy dicts on merge commits

[Yuya Nishihara]

>      while work:
> -        r, i1, copies1 = heapq.heappop(work)
> +        r, i1, copies = heapq.heappop(work)
>          if work and work[0][0] == r:
>              # We are tracing copies from both parents
>              r, i2, copies2 = heapq.heappop(work)
> -            copies = {}
> -            allcopies = set(copies1) | set(copies2)
> -            for dst in allcopies:
> +            for dst, src in copies2.items():
>                  # Unlike when copies are stored in the filelog, we consider
>                  # it a copy even if the destination already existed on the
>                  # other branch. It's simply too expensive to check if the
>                  # file existed in the manifest.
> -                if dst in copies1:
> -                    # If it was copied on the p1 side, mark it as copied from
> +                if dst not in copies:
> +                    # If it was copied on the p1 side, leave it as copied from
>                      # that side, even if it was also copied on the p2 side.
> -                    copies[dst] = copies1[dst]
> -                else:
>                      copies[dst] = copies2[dst]

Are we sure there's no `copies` alias held by later `work`?

I'm just asking because we've optimized some `copies.copy()`s away at
5ceb91136ebe. I don't know if it's safe or not to mutate `copies` at this
point.