git_ui: Introduce explicit yield points for project diff (#43706)

These are long running foreground tasks that tend to not have pending
await points, meaning once we start executing them they will never
reschedule themselves, starving the foreground thread.

Release Notes:

- Improved git project diff responsiveness

crates/git_ui/src/project_diff.rs

@@ -32,6 +32,7 @@ use project::{
     },
 };
 use settings::{Settings, SettingsStore};
+use smol::future::yield_now;
 use std::any::{Any, TypeId};
 use std::ops::Range;
 use std::sync::Arc;
@@ -584,6 +585,9 @@ impl ProjectDiff {
 
         for (entry, path_key) in buffers_to_load.into_iter().zip(path_keys.into_iter()) {
             if let Some((buffer, diff)) = entry.load.await.log_err() {
+                // We might be lagging behind enough that all future entry.load futures are no longer pending.
+                // If that is the case, this task will never yield, starving the foreground thread of execution time.
+                yield_now().await;
                 cx.update(|window, cx| {
                     this.update(cx, |this, cx| {
                         this.register_buffer(path_key, entry.file_status, buffer, diff, window, cx)