An object that encapsulates differs and predicates, and also the new "is_atomic", which is added so that we can mark e.g. cell IDs as atomic. DiffConfig could potentially also be used in the future to  better config notebook ignores etc?

When comparing identical sources we can avoid a full diff by a quick equality check.

Avoid recursing for string mimetypes that are identical

In the strictest check, cells without ID is never considered equal to cells with IDs, but they can be in the two less-strict comparisons. I.e. if a notebook ends up with only some cells having IDs, and then IDs are added to those cells later, then in the first pass the existing cells that all had IDs are first matched to each other, and only in the next passes are the cells that gained IDs compared with their old ID-less versions.

Always use line based diffing for source.

Enable previously not working test

This allows union strategy to correctly be applied. We might want to consider to silently ignore the strategy for dicts instead of raising, but it is hard to decide the best course without any good cases to consider.

With union strategy, this test can pass. It is probably correct for it to fail without the explicit strategy marked.

Only show it if `details` flag is set, and display it like any other field.

Now, we use cell IDs as the highest ranking comparison, and any cells that map equal will take priority in computing snakes of matching cells. Any further comparisons (strict, moderate, approximate) will be as before, but with the implication that the cells being compared do not have an ID match on the other side, so considering ID has no meaning.

Remove some skips that are no longer needed, fix tests that were marked xfail due to not being completed, and split some tests that had some hard-coded logical branches (`if 0:`) - fixing the previously disabled branch.

Good for collecting debug data on merge decisions.

Adds tests for the recently added ability to output merge decisions as raw JSON. As the tests indicate, this is a breaking behavior, but this is a major version release, so should be ok.

This is needed to have download test work (assumes no conflicts).

Click should only register as final after mouse up with primary button.