% git checkout -b upstream upstream-repo/upstream
warning: You appear to be on a branch yet to be born.
warning: Forcing checkout of upstream-repo/upstream.
Branch upstream set up to track remote branch refs/remotes/upstream-repo/upstream.
Switched to a new branch "upstream"

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% git checkout -b upstream upstream-repo/master
git checkout: updating paths is incompatible with switching branches/forcing
Did you intend to checkout 'upstream-repo/master' which can not be resolved as commit?