A linked git worktree with an unborn HEAD (a fresh/orphan branch with no
commits) still shares the parent's common dir, so an isolated task's
first branch and commit would write into the parent repo. The previous
early return on a missing HEAD SHA left that shared metadata intact.
detachGitDir now severs unborn worktrees too: `git init -b <branch>`
preserves the checked-out branch name, ref freezing is gated on a born
HEAD, and the rcopy worktree registration is still removed.
Fixes#6003
roboomp
37304a12a3