I had the following in my repo
Next-->Commit A.1,Commit A.2,Commit A.3 --......
git rebase -i origin next
You just need to do:
git rebase -i <COMMIT-ISH>
<COMMIT-ISH> is some way of referring to the commit marked
Master in your diagram. If there's a branch pointing to that point (probably
git log --decorate will show that.