Re: how to rebase backwards

Thank you for detail explanation.  From what you described, I begin
with master commit D, 
 $ git checkout -b deploy
 $ git commit --allow-empty -m deploy

 
                      E
                     /^ deploy
     ---A---B---C---D
                    ^ master
 
 $ git rebase -i A

                      v deploy
          B'--C'--D'--E'
         /
     ---A---B---C---D
                    ^ master

since E is an empty commit, I suppose content of D' E' and D are identical at this point. Is that correct?

If several months later, I forget which is the common ancestor for master and deploy, how do I generate the above graph or identify commit A as the common ancestor for these two branches?