If you use the default merge messages, can't you tell which was the branch that got merged? The second parent is the branch that got merged and its name will appear in the commit message on the merge commit. This is probably something that you could infer most of the time.
Git will provide you with a default message for git merge without specifying any flags. This message includes the branch being merged. I don't think there is support in git to view history with the branches being marked explicitly. However, you can get something similar with:
git log --pretty --oneline --graph --topo-order
That should at least group commits roughly by branch. Did that answer what you were asking?
