git branch -r
I guess one usage is that it can filter out unused/obsolete branches (that have very commits) and tidy up the repo
That is what you see in the "branches" tab section of a GitHub project:
Example for git/git/branches:
What you want is not the number of commits, but, for a given branch acting as reference, the number of commit ahead and behind that branch.
git rev-list --left-right --count master...test-branch
That way you can see the one behind that could be safely removed.
Note that you can already list those merged branches with:
git branch --merged master
With Git 2.5, you can also list local branches compared to their remote tracking branches (not your case, but can be useful):
git fetch git for-each-ref --format="%(push:track)" refs/heads