Fix ordering of multibuffer excerpts (#39476)

The ordering of path-based excerpts in multibuffers regressed with
#38744, because we changed the `path` field of `PathKey` to be a string
(from `std::path::Path`) and used the derived `Ord` implementation,
which doesn't agree with the path-based order of worktree traversals.
This PR fixes that by using `RelPath` for `PathKey`. Instead of using
`File::full_path`, which can be absolute, we always use `File::path` and
distinguish different worktrees using their ID.

Release Notes:

- N/A

---------

Co-authored-by: Lukas Wirth <me@lukaswirth.dev>

crates/editor/src/editor_tests.rs

@@ -16515,7 +16515,7 @@ async fn test_following_with_multiple_excerpts(cx: &mut TestAppContext) {
     leader.update(cx, |leader, cx| {
         leader.buffer.update(cx, |multibuffer, cx| {
             multibuffer.set_excerpts_for_path(
-                PathKey::namespaced(1, "b.txt".into()),
+                PathKey::namespaced(1, rel_path("b.txt").into_arc()),
                 buffer_1.clone(),
                 vec![
                     Point::row_range(0..3),
@@ -16526,7 +16526,7 @@ async fn test_following_with_multiple_excerpts(cx: &mut TestAppContext) {
                 cx,
             );
             multibuffer.set_excerpts_for_path(
-                PathKey::namespaced(1, "a.txt".into()),
+                PathKey::namespaced(1, rel_path("a.txt").into_arc()),
                 buffer_2.clone(),
                 vec![Point::row_range(0..6), Point::row_range(8..12)],
                 0,
@@ -21025,10 +21025,7 @@ async fn test_display_diff_hunks(cx: &mut TestAppContext) {
         for buffer in &buffers {
             let snapshot = buffer.read(cx).snapshot();
             multibuffer.set_excerpts_for_path(
-                PathKey::namespaced(
-                    0,
-                    buffer.read(cx).file().unwrap().path().as_unix_str().into(),
-                ),
+                PathKey::namespaced(0, buffer.read(cx).file().unwrap().path().clone()),
                 buffer.clone(),
                 vec![text::Anchor::MIN.to_point(&snapshot)..text::Anchor::MAX.to_point(&snapshot)],
                 2,