merge-package should maybe vary which branch it merges the combined upstream to

[James Westby]

When merge-upstream finds that it is merging different upstream
versions it does extra work to consolidate the upstreams first.

It creates a revision with the contents of the latest of the two
upstreams, which has both of them as parents, and then merges
this in to the target branch.

It should perhaps merge this in to the side which has the latest
upstream.

With the current strategy you can get conflicts on that part
of the operation, which are confusing to the user, and perhaps
spurious.

If we change that then the conflicts cannot arise at that point, so
as long as you would still get conflicts in the right places on
the final merge this would give a better user experience.

I'm not confident that I have considered all the cases, so just
filing a bug for now.