git: More performance improvements when toggling between diff views (#49400)

- Defer syncing block maps from `set_companion`, eliminating some
redundant recomputations
- Emit one large multibuffer edit from `set_show_deleted_hunks` instead
of many small edits, to avoid bad case for block map

This cuts hangs roughly in half when toggling between views in a large
diff (1000 commits from the chromium repository).

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Release Notes:

- Improved performance with large diffs when toggling between diff
views.

---------

Co-authored-by: Jakub Konka <kubkon@jakubkonka.com>
Co-authored-by: Lukas Wirth <lukas@zed.dev>

crates/multi_buffer/src/multi_buffer.rs

@@ -2710,7 +2710,25 @@ impl MultiBuffer {
 
     pub fn set_show_deleted_hunks(&mut self, show: bool, cx: &mut Context<Self>) {
         self.snapshot.get_mut().show_deleted_hunks = show;
-        self.expand_or_collapse_diff_hunks(vec![Anchor::min()..Anchor::max()], true, cx);
+
+        self.sync_mut(cx);
+
+        let old_len = self.snapshot.borrow().len();
+
+        let ranges = std::iter::once((Point::zero()..Point::MAX, ExcerptId::max()));
+        let _ = self.expand_or_collapse_diff_hunks_inner(ranges, true, cx);
+
+        let new_len = self.snapshot.borrow().len();
+
+        self.subscriptions.publish(vec![Edit {
+            old: MultiBufferOffset(0)..old_len,
+            new: MultiBufferOffset(0)..new_len,
+        }]);
+
+        cx.emit(Event::DiffHunksToggled);
+        cx.emit(Event::Edited {
+            edited_buffer: None,
+        });
     }
 
     pub fn set_use_extended_diff_range(&mut self, use_extended: bool, _cx: &mut Context<Self>) {
@@ -2768,9 +2786,9 @@ impl MultiBuffer {
         ranges: impl IntoIterator<Item = (Range<Point>, ExcerptId)>,
         expand: bool,
         cx: &mut Context<Self>,
-    ) {
+    ) -> Vec<Edit<MultiBufferOffset>> {
         if self.snapshot.borrow().all_diff_hunks_expanded && !expand {
-            return;
+            return Vec::new();
         }
         self.sync_mut(cx);
         let mut snapshot = self.snapshot.get_mut();
@@ -2796,18 +2814,11 @@ impl MultiBuffer {
             }
         }
 
-        let edits = Self::sync_diff_transforms(
+        Self::sync_diff_transforms(
             &mut snapshot,
             excerpt_edits,
             DiffChangeKind::ExpandOrCollapseHunks { expand },
-        );
-        if !edits.is_empty() {
-            self.subscriptions.publish(edits);
-        }
-        cx.emit(Event::DiffHunksToggled);
-        cx.emit(Event::Edited {
-            edited_buffer: None,
-        });
+        )
     }
 
     pub fn expand_or_collapse_diff_hunks(
@@ -2826,7 +2837,14 @@ impl MultiBuffer {
             };
             (range.start..peek_end, end_excerpt_id)
         });
-        self.expand_or_collapse_diff_hunks_inner(ranges, expand, cx);
+        let edits = self.expand_or_collapse_diff_hunks_inner(ranges, expand, cx);
+        if !edits.is_empty() {
+            self.subscriptions.publish(edits);
+        }
+        cx.emit(Event::DiffHunksToggled);
+        cx.emit(Event::Edited {
+            edited_buffer: None,
+        });
     }
 
     pub fn resize_excerpt(
@@ -3629,6 +3647,22 @@ impl MultiBuffer {
         );
         did_extend
     }
+
+    pub fn toggle_single_diff_hunk(&mut self, range: Range<Anchor>, cx: &mut Context<Self>) {
+        let snapshot = self.snapshot(cx);
+        let excerpt_id = range.end.excerpt_id;
+        let point_range = range.to_point(&snapshot);
+        let expand = !self.single_hunk_is_expanded(range, cx);
+        let edits =
+            self.expand_or_collapse_diff_hunks_inner([(point_range, excerpt_id)], expand, cx);
+        if !edits.is_empty() {
+            self.subscriptions.publish(edits);
+        }
+        cx.emit(Event::DiffHunksToggled);
+        cx.emit(Event::Edited {
+            edited_buffer: None,
+        });
+    }
 }
 
 fn build_excerpt_ranges(