The only solution I can think of is to write a prepare-commit-msg hook that adds a line like "On branch: <branch-name>" to the commit message. So when reading your commit history, every commit that was made on this branch would contain this message. You can also look up just commits made on a particular branch by doing `git log --grep="On branch: <branch-name>"` this way.
At the very least, it may be useful as a starting point.
While I use it extensively on large projects, I find that the merge commit can do just as well. Of course, that doesn't help you outside context of the merge commit---bisecting, for example---unless you are okay with discovering the merge commit that introduced it into the mainline. That can still be scripted.
