Before the patch, try something like `arc work T12345` (e.g. `T15993` that exists here in Phorge) and see that a TODO exception is raised.
After the patch, try again, and you see that a new branch is created:
BRANCH Checking out branch "T15993-support-for-arc-work-t12345-(workontask-workflow)".
Run git status to double-check if you are in the mentioned branch.
Run the command again, and you are still in the correct branch. So, the system recognizes that a branch already exist and use that again.