`jit new remote:branch` fails if `branch` already exists

[barrettj12]

$ jit new juju:3.1 --no-edit
creating new worktree based on remote branch "juju:3.1"
Error: failed to create new branch: couldn't create branch "3.1": fatal: a branch named '3.1' already exists

exit status 128

If the branch already exists, we should just create a worktree and check out the local branch.