Let’s say you have setup these branches:

A
 \
   B --> C
    \
     D

Sometimes someone coding in branch C will finish it’s development, merge to B and rebase iteractely, creating a new branch of itself and kind leaving D without a real parent branch (not exactly since B still exists in the reflog):

A
 \
  B'

  D

The issue been faced here is that D parent should be B' now, if we want to be able to merge in a fast-forward manner and avoid unnecessary conflicts, we need to fix this. Here is a way to do it:

git branch -m D D-old
git checkout -b D B'
git cherry-pick <All-commits-from old D>
git push -f origin D

We are just getting our commits from old-D into a new D which is based on B'. This may sound unnecessary but when you have a pull request, we don’t want to close it and lose all comments/history of the pull request, this does the trick.