When checking out a repo and specifying a non-master branch, the existing checkout command fails if the repo uses that branch as its base. (Ex: git clone of a repo that uses mybranch as the base (default) instead of master would already be on the mybranch branch from git clone. Then git checkout -b mybranch … would fail with "fatal: A branch named 'mybranch' already exists.")

For example: cap deploy branch=different_from_current