help reproduce fast-forward in rebase

Hi,

I'm rewriting rebase in C and do not quite understand this code inside git-rebase.sh

# If the $onto is a proper descendant of the tip of the branch, then
# we just fast forwarded.
if test "$mb" = "$branch"
then
	echo >&2 "Fast-forwarded $branch_name to $onto_name."
	move_to_original_branch
	exit 0
fi

Anyone has an example how to make "git rebase" execute that code?