Here's the current branch setup:

```
A---B---C-----------------D---E---F master
         \
          G---H---I---J---K---L branchA

```

I realized when I went to work on branchA, that the last few commits were for a new feature that should be a new branch. So I want to get it such that the tree looks like:

```
A---B---C-----------------D---E---F master
         \
          G---H---I------------M branchA
                   \
                    J---K---L branchB

```

I don't want to rename the branch, since that would rename G, H, and I to the new branch name. I can create a new branch, starting at I (`git branch branchB <SHA-of-I>`), but how best to "move" commits J, K, and L over? `git merge branchA` while on branchB just fast-forwards the branchB head to the same point as branchA.