[PATCH 2 of 7] copies: update _checkcopies to work in a rotated DAG

[timeless]

Gábor Stefanik wrote:
> copies: update _checkcopies to work in a rotated DAG

I'd argue this should be tagged (BC).
I absolutely support this change, it should be changed, it is a bug.

But the reason I'd argue that it's a change is that a repository
constructed along this path will result in a different changeset
depending on the version of hg:

> @@ -620,7 +620,7 @@
>    date:        Thu Jan 01 00:00:00 1970 +0000
>    summary:     2
>
> -  changeset:   14:f64defefacee
> +  changeset:   14:0c921c65ef1e
>    parent:      1:5d205f8b35b6
>    user:        foo
>    date:        Thu Jan 01 00:00:00 1970 +0000