Rebasing cascading topic trees

[Norbert Kiesel <nkiesel@gmail.com>]

I currently have a situation with cascading topic branches that I need to rebase
regularly.  In the picture below, I want to rebase the tree starting with `E` to
be rebased onto master (my actually cascade is 4 branches deep).

A--B--C--D (master)
   \
    E--F (topic1)
       \
        G--H (topic2)

After running `git rebase --onto master master topic1`, I end up with

A--B--C--D (master)
   |     \
   \      E'--F' (topic1)
    E--F
       \
        G--H (topic2)

I then need to also run `git rebase --onto topic1 F topic2` to arrive at the
desired

A--B--C--D (master)
   |     \
   \      E'--F' (topic1)
    E--F      \
       |       G'--H' (topic2)
       \
        G--H

Problem here is that I don't have a nice symbolic name for `F` anymore after the
first rebase. Rebasing `topic2` first is not really possible, because I do not
have a new graft-point yet.  I currently write down `F` ahead of time (or use
`reflog` if I forgot) `F`, but I wonder if there is a better solution.