add `--ff` to `git merge` invocations

Merges don't work correctly when merge.ff is set to only in Git's configuration. Fix this by specifying --ff, which does what git-assembler seems to intend (see also upstream docs):

With --ff, when possible resolve the merge as a fast-forward (only update the branch pointer to match the merged branch; do not create a merge commit). When not possible (when the merged-in history is not a descendant of the current history), create a merge commit.