Use `Fs` instead of `std::fs` and do entry existence checks better:
* first, check the worktree entry existence without any FS checks
* then, only for local cases, use `Fs` to check for abs_path existence
of items, in case those came from single-filed worktrees that got closed
and removed.
Remote entries do not get file existence checks, so might try opening
previously removed buffers for now.
Release Notes:
- N/A
454d3dd52b