Function worktree_gitdir_resolves 

fn worktree_gitdir_resolves(path: &Path) -> bool

Whether the worktree registered at path still points at a gitdir git can reuse.

git reads several facts when it decides to reuse or refuse a worktree, so the link alone is not the question: the .git entry must be the plain link file git writes (not a symlink standing in for one), the gitdir it names must be there, that gitdir’s own gitdir back-link must name THIS worktree’s .git entry, and its HEAD must resolve. Each of those states is one git’s own reuse probe returns null for, and the add then refuses - the silent failure this check exists to catch (#1593).