Highlight merge conflicts and provide for resolving them (#28065)

TODO:

- [x] Make it work in the project diff:
  - [x] Support non-singleton buffers
  - [x] Adjust excerpt boundaries to show full conflicts
- [x] Write tests for conflict-related events and state management
- [x] Prevent hunk buttons from appearing inside conflicts
- [x] Make sure it works over SSH, collab
- [x] Allow separate theming of markers

Bonus:

- [ ] Count of conflicts in toolbar
- [ ] Keyboard-driven navigation and resolution
- [ ] ~~Inlay hints to contextualize "ours"/"theirs"~~

Release Notes:

- Implemented initial support for resolving merge conflicts.

---------

Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>

crates/multi_buffer/src/multi_buffer.rs

@@ -95,6 +95,7 @@ pub enum Event {
     },
     ExcerptsRemoved {
         ids: Vec<ExcerptId>,
+        removed_buffer_ids: Vec<BufferId>,
     },
     ExcerptsExpanded {
         ids: Vec<ExcerptId>,
@@ -2021,7 +2022,12 @@ impl MultiBuffer {
     pub fn clear(&mut self, cx: &mut Context<Self>) {
         self.sync(cx);
         let ids = self.excerpt_ids();
-        self.buffers.borrow_mut().clear();
+        let removed_buffer_ids = self
+            .buffers
+            .borrow_mut()
+            .drain()
+            .map(|(id, _)| id)
+            .collect();
         self.excerpts_by_path.clear();
         self.paths_by_excerpt.clear();
         let mut snapshot = self.snapshot.borrow_mut();
@@ -2046,7 +2052,10 @@ impl MultiBuffer {
             singleton_buffer_edited: false,
             edited_buffer: None,
         });
-        cx.emit(Event::ExcerptsRemoved { ids });
+        cx.emit(Event::ExcerptsRemoved {
+            ids,
+            removed_buffer_ids,
+        });
         cx.notify();
     }
 
@@ -2310,9 +2319,9 @@ impl MultiBuffer {
         new_excerpts.append(suffix, &());
         drop(cursor);
         snapshot.excerpts = new_excerpts;
-        for buffer_id in removed_buffer_ids {
-            self.diffs.remove(&buffer_id);
-            snapshot.diffs.remove(&buffer_id);
+        for buffer_id in &removed_buffer_ids {
+            self.diffs.remove(buffer_id);
+            snapshot.diffs.remove(buffer_id);
         }
 
         if changed_trailing_excerpt {
@@ -2325,7 +2334,10 @@ impl MultiBuffer {
             singleton_buffer_edited: false,
             edited_buffer: None,
         });
-        cx.emit(Event::ExcerptsRemoved { ids });
+        cx.emit(Event::ExcerptsRemoved {
+            ids,
+            removed_buffer_ids,
+        });
         cx.notify();
     }
 

crates/multi_buffer/src/multi_buffer_tests.rs

@@ -635,7 +635,7 @@ fn test_excerpt_events(cx: &mut App) {
                     predecessor,
                     excerpts,
                 } => follower.insert_excerpts_with_ids_after(predecessor, buffer, excerpts, cx),
-                Event::ExcerptsRemoved { ids } => follower.remove_excerpts(ids, cx),
+                Event::ExcerptsRemoved { ids, .. } => follower.remove_excerpts(ids, cx),
                 Event::Edited { .. } => {
                     *follower_edit_event_count.write() += 1;
                 }