Rebase feature branch onto another feature branch

1. Switch to Branch2

   git checkout Branch2
   

2. Apply the current (Branch2) changes on top of the Branch1 changes, staying
   in Branch2:

   git rebase Branch1
   

Which would leave you with the desired result in Branch2:

a -- b -- c                      <-- Master
           \
            d -- e               <-- Branch1
           \
            d -- e -- f' -- g'   <-- Branch2

You can delete Branch1.