Add support for merging Changes objects

This works in a similar way to the mergechanges program in devscripts, although the implementation is much simpler since Deb822.merge_fields does a good deal of the work.

I initially did this in freexian-team/debusine!1765 (merged), but Enrico pointed out that something similar might reasonably be submitted here. I'm happy for us to maintain our version in the meantime though.