sum_tree: Replace rayon with futures (#41586)

Release Notes:

- N/A *or* Added/Fixed/Improved ...

Co-authored by: Kate <kate@zed.dev>

crates/git_ui/src/file_diff_view.rs

@@ -359,6 +359,7 @@ mod tests {
     use super::*;
     use editor::test::editor_test_context::assert_state_with_diff;
     use gpui::TestAppContext;
+    use language::Rope;
     use project::{FakeFs, Fs, Project};
     use settings::SettingsStore;
     use std::path::PathBuf;
@@ -429,7 +430,7 @@ mod tests {
         // Modify the new file on disk
         fs.save(
             path!("/test/new_file.txt").as_ref(),
-            &unindent(
+            &Rope::from_str_small(&unindent(
                 "
                 new line 1
                 line 2
@@ -437,8 +438,7 @@ mod tests {
                 line 4
                 new line 5
                 ",
-            )
-            .into(),
+            )),
             Default::default(),
         )
         .await
@@ -465,15 +465,14 @@ mod tests {
         // Modify the old file on disk
         fs.save(
             path!("/test/old_file.txt").as_ref(),
-            &unindent(
+            &Rope::from_str_small(&unindent(
                 "
                 new line 1
                 line 2
                 old line 3
                 line 4
                 ",
-            )
-            .into(),
+            )),
             Default::default(),
         )
         .await