diff: use object diff #6594
Description
Currently dvc/repo/diff.py is quite a bit outdated, we didn't touch it since before introducing objects, so it is still based on hash_info, which makes it suboptimal and very confusing. What we should do instead is introduce dvc/objects/diff.py and implement generic diffing logic for Tree objects, that could then be used in dvc diff. Also it is important to remember that even though right now we don't provide one Tree for the whole repo (kinda like git commits do), we will do that soon, which will again simplify dvc diff logic, that will then use objects.diff even more directly.
Btw, we are already doing basic object diff when merging trees in our git merge-driver (see merge in dvc/objects/tree.py).