The merge function does the merge (or in this case tells you you could skip it), but it's up to you (or the user of the tool) whether you want to move the current branch to the new position.
Doing that is the same as any other time you want to change a reference. In this case you want to get to the current branch, which you do via resolving HEAD
to a non-symbolic reference and setting its target.
repo.lookup_reference('HEAD').resolve().target = merge_result.fastforward_oid