Revert "sum_tree: Replace rayon with futures (#41586) (#41846)

This causes the background executor to hang

Release Notes:

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

crates/git_ui/src/file_diff_view.rs

@@ -359,7 +359,6 @@ 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;
@@ -430,7 +429,7 @@ mod tests {
         // Modify the new file on disk
         fs.save(
             path!("/test/new_file.txt").as_ref(),
-            &Rope::from_str_small(&unindent(
+            &unindent(
                 "
                 new line 1
                 line 2
@@ -438,7 +437,8 @@ mod tests {
                 line 4
                 new line 5
                 ",
-            )),
+            )
+            .into(),
             Default::default(),
         )
         .await
@@ -465,14 +465,15 @@ mod tests {
         // Modify the old file on disk
         fs.save(
             path!("/test/old_file.txt").as_ref(),
-            &Rope::from_str_small(&unindent(
+            &unindent(
                 "
                 new line 1
                 line 2
                 old line 3
                 line 4
                 ",
-            )),
+            )
+            .into(),
             Default::default(),
         )
         .await