In a Branch list plugin I had knocked up myself, I made the behind/ahead numbers clickable towards the commit page, this will clearly show which X/Y commits the behind/ahead git is talking about.

Basically you can point the soy template to the repo/commits?until=branch1..branch2 and reverse it for the other link, it will just give you a git diff of the two branches which is exactly what we wanna see.

I also added a filter checkbox on the commit window, which filters out all merges (instead of showing them gray), so you get a clear view of the branch diffs, a nice plus.