Dependent pull requests

While working on two features A and B it may happen that work on feature B to go incrementally or to be dependent of work on feature A.

We have to branches, A and B initiated from a common development branch, let's say it is master in this case.
Ar some point in time, while working on branch B we realize that the work done on branch A is useful and merge branch A in branch B, knowing that A will be ready and merged in master before B will.

Code reviewing feature A works great. You have the source A, the destination master.
When reviewing work on branch B, the destination is again master but the Diff tab will show all the difference both branches A and B introduces against master.
We need to review only new changes in B.

When creating/editing a review we need a box to select other reviews that should be dependent on.
While considering commits to review it is all commits in branch B and not in master or branch A.
To show the diff it should be the diff introduced by the merge of branch B in master, after branch A is already merged.