Take the following case:
I have some work in a topic branch and now I\'m ready to merge back to master:
* eb3b733 3 [master] [origin/master]
| * b6
It looks like what you want to do is remove your first merge. You could follow the following procedure:
git checkout master # Let's make sure we are on master branch
git reset --hard master~ # Let's get back to master before the merge
git pull # or git merge remote/master
git merge topic
That would give you what you want.