gistfile1.txt

Prepare: 
 mkdir foo
 cd foo
 git init
 echo A>A
 echo B>B
 git add A B
 git commit -m AB
 git checkout -b br1
 echo BB>>B
 git commit -a -m BB
 git checkout -b br2 master
 echo AA>>A
 git commit -a -m AA
 git branch br2_orig
 git checkout master
 git rebase --onto master master br1
 git checkout master
 git rebase br1
 git rebase --onto master master br2
 git checkout master
 git rebase br2
User then continues extending his original br2 (called br2_orig) and makes another pull request: 
 git checkout br2_orig
 echo AAA>>A
 git commit -a -m AAA
 git checkout master
 git rebase --onto master master br2_orig
 git checkout master
 git rebase br2_orig