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/multi_buffer/src/multi_buffer_tests.rs

@@ -8,6 +8,7 @@ use rand::prelude::*;
 use settings::SettingsStore;
 use std::env;
 use util::RandomCharIter;
+use util::rel_path::rel_path;
 use util::test::sample_text;
 
 #[ctor::ctor]
@@ -1524,7 +1525,7 @@ fn test_set_excerpts_for_buffer_ordering(cx: &mut TestAppContext) {
             cx,
         )
     });
-    let path1: PathKey = PathKey::namespaced(0, "/".into());
+    let path1: PathKey = PathKey::namespaced(0, rel_path("root").into_arc());
 
     let multibuffer = cx.new(|_| MultiBuffer::new(Capability::ReadWrite));
     multibuffer.update(cx, |multibuffer, cx| {
@@ -1619,7 +1620,7 @@ fn test_set_excerpts_for_buffer(cx: &mut TestAppContext) {
             cx,
         )
     });
-    let path1: PathKey = PathKey::namespaced(0, "/".into());
+    let path1: PathKey = PathKey::namespaced(0, rel_path("root").into_arc());
     let buf2 = cx.new(|cx| {
         Buffer::local(
             indoc! {
@@ -1638,7 +1639,7 @@ fn test_set_excerpts_for_buffer(cx: &mut TestAppContext) {
             cx,
         )
     });
-    let path2 = PathKey::namespaced(1, "/".into());
+    let path2 = PathKey::namespaced(1, rel_path("root").into_arc());
 
     let multibuffer = cx.new(|_| MultiBuffer::new(Capability::ReadWrite));
     multibuffer.update(cx, |multibuffer, cx| {
@@ -1815,7 +1816,7 @@ fn test_set_excerpts_for_buffer_rename(cx: &mut TestAppContext) {
             cx,
         )
     });
-    let path: PathKey = PathKey::namespaced(0, "/".into());
+    let path: PathKey = PathKey::namespaced(0, rel_path("root").into_arc());
     let buf2 = cx.new(|cx| {
         Buffer::local(
             indoc! {