Your comments

OMG, it turned out I didn't use the 19.1 RC. It works exactly as I expected. Brilliant! Thanks a lot!

This is a good solution. But I was talking about more general approach, where you can expand merge commits.

It would be really often to be able to expand a certain merge on click. E.g. I look through the history, notice an interesting merge, and I want to "go deeper". I click on it, and then the merged branch expands! Magic!

This is absolutely awesome! I really love it. Can I make a minor suggestion though? It would be convenient and consistent to draw a little "tail" for merge commits, just the way it is done in Journal.

You can borrow some inspiration in Plastic SCM, how they do Branches View. Basically, each branch is considered a detail. Initially you can see only main line. You see merges on it. Interested in details? Expand a merge, look what's in it.