git: Implement branch diff line counts efficiently (#52582)

Previously we were iterating over all hunks across all diffs on every
frame. Now we can read off the required information as a `SumTree`
summary in constant time.

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [ ] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- git: Fixed bad performance in large branch diffs.

crates/git_ui/src/commit_view.rs

@@ -414,38 +414,7 @@ impl CommitView {
     }
 
     fn calculate_changed_lines(&self, cx: &App) -> (u32, u32) {
-        let snapshot = self.multibuffer.read(cx).snapshot(cx);
-        let mut total_additions = 0u32;
-        let mut total_deletions = 0u32;
-
-        let mut seen_buffers = std::collections::HashSet::new();
-        for (_, buffer, _) in snapshot.excerpts() {
-            let buffer_id = buffer.remote_id();
-            if !seen_buffers.insert(buffer_id) {
-                continue;
-            }
-
-            let Some(diff) = snapshot.diff_for_buffer_id(buffer_id) else {
-                continue;
-            };
-
-            let base_text = diff.base_text();
-
-            for hunk in diff.hunks_intersecting_range(Anchor::MIN..Anchor::MAX, buffer) {
-                let added_rows = hunk.range.end.row.saturating_sub(hunk.range.start.row);
-                total_additions += added_rows;
-
-                let base_start = base_text
-                    .offset_to_point(hunk.diff_base_byte_range.start)
-                    .row;
-                let base_end = base_text.offset_to_point(hunk.diff_base_byte_range.end).row;
-                let deleted_rows = base_end.saturating_sub(base_start);
-
-                total_deletions += deleted_rows;
-            }
-        }
-
-        (total_additions, total_deletions)
+        self.multibuffer.read(cx).snapshot(cx).total_changed_lines()
     }
 
     fn render_header(&self, window: &mut Window, cx: &mut Context<Self>) -> impl IntoElement {