when merging over an array, concat

[fiatjaf]

See tests for an explanation much better than what I could write here.

[trentm]

Thanks for the great test cases. Unfortunately I don't want this change. This would change behaviour of --merge so would break compat.

At the least it would require a separate option for this different kind of merging, say --merge-concat. However, I don't have a feel for how valuable this option would be. If you had some real world example that would help a bit. Then if some other people could claim real world uses for this, I'd consider adding it.