Support creating/viewing diffs from local files

[naoliv]

Is it possible to support uploading local files and then create/render a diff from them, please? For example, suppose I see one user with 1000 small changesets and I would like to review them; doing this one-by-one with achavi would be inefficient (not due to achavi, but due to the amount of changesets with small changes).

Now suppose that I download all of his osmChanges, merge them with osmium merge-changes -F osc -f osm -o merged.osc <all his osmchanges.osc> and open this local file with the achavi interface. We would be able to view the before and after of all his 1000 changesets, in just one single page.

This would help a lot (really a lot) with things like https://github.com/openstreetmap/iD/issues/2251#issuecomment-180469767 and https://lists.openstreetmap.org/pipermail/talk/2018-January/080040.html

Could it be possible to do something like this? :-)