In my test case I have 3 commits and do actual merge. Tree looks like this
init - - - a  < - master
   \        \
    b - - - d  < - merge
    |
  side

In testFastForwardWithFiles there are 2 commits and no actual merge. Tree looks like this

   a   - - -  b  <- master
   |          |
branch1  branch1
before    after
      merge