git_ui: Fix open diff for untracked files when sorting by path enabled (#39862)

Fixes the `Open Diff` action for untracked files when the `sort_by_path`
setting is enabled. The `ProjectDiff` wasn't correctly moving the
multibuffer's cursor to the untracked file because, when that setting is
enabled, it's sort prefix is changed to the tracked files sort prefix, and that
wasn't accounted for in `move_to_entry`.

Before these changes, the `sort_prefix` field for `PathKey` was called `namespace`, it was renamed to be clearer what its purpose is.

Closes #39529 

Release Notes:

- Fixed 'Open Diff' action for untracked files when `sort_by_path` is
enabled

---------

Co-authored-by: David Kleingeld <davidsk@zed.dev>

crates/editor/src/editor_tests.rs

@@ -16518,7 +16518,7 @@ 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, rel_path("b.txt").into_arc()),
+                PathKey::with_sort_prefix(1, rel_path("b.txt").into_arc()),
                 buffer_1.clone(),
                 vec![
                     Point::row_range(0..3),
@@ -16529,7 +16529,7 @@ async fn test_following_with_multiple_excerpts(cx: &mut TestAppContext) {
                 cx,
             );
             multibuffer.set_excerpts_for_path(
-                PathKey::namespaced(1, rel_path("a.txt").into_arc()),
+                PathKey::with_sort_prefix(1, rel_path("a.txt").into_arc()),
                 buffer_2.clone(),
                 vec![Point::row_range(0..6), Point::row_range(8..12)],
                 0,
@@ -21032,7 +21032,7 @@ 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().clone()),
+                PathKey::with_sort_prefix(0, buffer.read(cx).file().unwrap().path().clone()),
                 buffer.clone(),
                 vec![text::Anchor::MIN.to_point(&snapshot)..text::Anchor::MAX.to_point(&snapshot)],
                 2,