git: fix `gbda` trying to delete worktree branches (#8102)

Git learned to add a `+` in front of branches that are
checked out in other worktrees.

See: https://github.com/git/git/blob/745f6812895b31c02b29bdfe4ae8e5498f776c26/Documentation/RelNotes/2.23.0.txt#L252-L256