Modified version of #25950. We still use worktree paths, but repo paths
with a status that lie outside the worktree are not excluded; instead,
we relativize them by adding `..`. This makes the list in the git panel
match what you'd get from running `git status` (with the repo's worktree
root as the working directory).

- [x] Implement + test new unrelativization logic
- [x] ~~When collecting repositories, dedup by .git abs path, so
worktrees can share a repo at the project level~~ dedup repos at the
repository selector layer, with repos coming from larger worktrees being
preferred
- [x] Open single-file worktree with diff when activating a path not in
the worktree

Release Notes:

- N/A