git branch -vv marks column 1 with '+' (not '*') for a branch checked
out in another linked worktree. Only '*' was stripped from $1, so a
gone branch shown with '+' left a bogus "+" entry in $gone_branches
that then failed to delete: error: branch '+' not found.
Add a regression case reproducing the '+'-marked gone-branch line via
the existing git-clean mock handler in tests/test-network-fish.fish.
rootiest