gh stack add should allow for adding existing branches

[marcofranssen]

I have been using stacked PRs for ages and use git aliases for a long time to do stacking and rebasing etc. Now I wanted to try the feature by adding existing stacked branches into a stack, but it seems I can't add exiting branches to the stack

git checkout my-branch
gh stack init
gh stack-add my-stacked-branch
branch "my-stacked-branch" already exists