Use diff summary for action log stats instead of iterating hunks (#61272)

The diff already has a constant-time accessor to get the counts of
added/removed rows, use that instead of iterating hunks on the
foreground.

Release Notes:
- Improved performance in agent threads with many file changes.

crates/action_log/src/action_log.rs

@@ -1047,23 +1047,12 @@ pub struct DiffStats {
 }
 
 impl DiffStats {
-    pub fn single_file(buffer: &Buffer, diff: &BufferDiff, cx: &App) -> Self {
-        let mut stats = DiffStats::default();
-        let diff_snapshot = diff.snapshot(cx);
-        let buffer_snapshot = buffer.snapshot();
-        let base_text = diff_snapshot.base_text();
-
-        for hunk in diff_snapshot.hunks(&buffer_snapshot) {
-            let added_rows = hunk.range.end.row.saturating_sub(hunk.range.start.row);
-            stats.lines_added += added_rows;
-
-            let base_start = hunk.diff_base_byte_range.start.to_point(base_text).row;
-            let base_end = hunk.diff_base_byte_range.end.to_point(base_text).row;
-            let removed_rows = base_end.saturating_sub(base_start);
-            stats.lines_removed += removed_rows;
+    pub fn single_file(diff: &BufferDiff) -> Self {
+        let (lines_added, lines_removed) = diff.changed_row_counts();
+        DiffStats {
+            lines_added,
+            lines_removed,
         }
-
-        stats
     }
 
     pub fn all_files(
@@ -1071,8 +1060,8 @@ impl DiffStats {
         cx: &App,
     ) -> Self {
         let mut total = DiffStats::default();
-        for (buffer, diff) in changed_buffers {
-            let stats = DiffStats::single_file(buffer.read(cx), diff.read(cx), cx);
+        for (_, diff) in changed_buffers {
+            let stats = DiffStats::single_file(diff.read(cx));
             total.lines_added += stats.lines_added;
             total.lines_removed += stats.lines_removed;
         }

crates/agent_ui/src/conversation_view/thread_view.rs

@@ -3215,7 +3215,7 @@ impl ThreadView {
                                     .size(IconSize::Small)
                             });
 
-                        let file_stats = DiffStats::single_file(buffer.read(cx), diff.read(cx), cx);
+                        let file_stats = DiffStats::single_file(diff.read(cx));
 
                         let buttons = self.render_edited_files_buttons(
                             index,

crates/buffer_diff/src/buffer_diff.rs

@@ -2122,6 +2122,12 @@ impl BufferDiff {
             .is_some_and(|diff_snapshot| diff_snapshot.base_text_exists)
     }
 
+    pub fn changed_row_counts(&self) -> (u32, u32) {
+        self.diff_snapshot
+            .as_ref()
+            .map_or((0, 0), |diff_snapshot| diff_snapshot.changed_row_counts())
+    }
+
     pub fn snapshot(&self, cx: &App) -> BufferDiffSnapshot {
         let mut snapshot = self.diff_snapshot.clone().unwrap_or_else(|| {
             let base_text = self.base_text_buffer.read(cx).snapshot();