Worktree paths in git panel, take 2 (#26047)

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

crates/worktree/src/worktree_tests.rs

@@ -3412,6 +3412,43 @@ async fn test_private_single_file_worktree(cx: &mut TestAppContext) {
     });
 }
 
+#[gpui::test]
+fn test_unrelativize() {
+    let work_directory = WorkDirectory::in_project("");
+    pretty_assertions::assert_eq!(
+        work_directory.try_unrelativize(&"crates/gpui/gpui.rs".into()),
+        Some(Path::new("crates/gpui/gpui.rs").into())
+    );
+
+    let work_directory = WorkDirectory::in_project("vendor/some-submodule");
+    pretty_assertions::assert_eq!(
+        work_directory.try_unrelativize(&"src/thing.c".into()),
+        Some(Path::new("vendor/some-submodule/src/thing.c").into())
+    );
+
+    let work_directory = WorkDirectory::AboveProject {
+        absolute_path: Path::new("/projects/zed").into(),
+        location_in_repo: Path::new("crates/gpui").into(),
+    };
+
+    pretty_assertions::assert_eq!(
+        work_directory.try_unrelativize(&"crates/util/util.rs".into()),
+        None,
+    );
+
+    pretty_assertions::assert_eq!(
+        work_directory.unrelativize(&"crates/util/util.rs".into()),
+        Path::new("../util/util.rs").into()
+    );
+
+    pretty_assertions::assert_eq!(work_directory.try_unrelativize(&"README.md".into()), None,);
+
+    pretty_assertions::assert_eq!(
+        work_directory.unrelativize(&"README.md".into()),
+        Path::new("../../README.md").into()
+    );
+}
+
 #[track_caller]
 fn check_git_statuses(snapshot: &Snapshot, expected_statuses: &[(&Path, GitSummary)]) {
     let mut traversal = snapshot