git: More rigorously test excerpt syncing for split diff, and fix a couple of bugs (#49907)

This PR adds a more rigorous test for the excerpt syncing logic in
`SplittableEditor`, in preparation for refactoring that code, since
we've had some bugs there.

The new test covers
- edits within the RHS multibuffer
- edits to the individual main buffers, not necessarily within the
excerpt bounds of the split diff
- excerpt expansion
- excerpt removal
- excerpt recalculation based on diff hunk ranges

Bugs fixed:
- incorrect edit merging logic in `patches_for_range`
- `merge_excerpts` leaving stale excerpt IDs in `excerpts_by_path`

Before you mark this PR as ready for review, make sure that you have:
- [x] Added a solid test coverage and/or screenshots from doing manual
testing
- [x] Done a self-review taking into account security and performance
aspects
- [ ] Aligned any UI changes with the [UI
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)

Release Notes:

- N/A

crates/multi_buffer/src/multi_buffer.rs

@@ -595,14 +595,12 @@ impl DiffState {
                             base_text_changed_range,
                             extended_range: _,
                         }) => {
-                            if let Some(base_text_changed_range) = base_text_changed_range.clone() {
-                                this.inverted_buffer_diff_changed(
-                                    diff,
-                                    main_buffer,
-                                    base_text_changed_range,
-                                    cx,
-                                )
-                            }
+                            this.inverted_buffer_diff_changed(
+                                diff,
+                                main_buffer,
+                                base_text_changed_range.clone(),
+                                cx,
+                            );
                             cx.emit(Event::BufferDiffChanged);
                         }
                         BufferDiffEvent::LanguageChanged => {
@@ -2202,7 +2200,19 @@ impl MultiBuffer {
         drop(snapshot);
 
         self.resize_excerpt(excerpt_ids[0], union_range, cx);
-        self.remove_excerpts(excerpt_ids[1..].iter().copied(), cx);
+        let removed = &excerpt_ids[1..];
+        for &excerpt_id in removed {
+            if let Some(path) = self.paths_by_excerpt.get(&excerpt_id) {
+                if let Some(excerpt_list) = self.excerpts_by_path.get_mut(path) {
+                    excerpt_list.retain(|id| *id != excerpt_id);
+                    if excerpt_list.is_empty() {
+                        let path = path.clone();
+                        self.excerpts_by_path.remove(&path);
+                    }
+                }
+            }
+        }
+        self.remove_excerpts(removed.iter().copied(), cx);
 
         excerpt_ids[0]
     }
@@ -2489,7 +2499,7 @@ impl MultiBuffer {
         &mut self,
         diff: Entity<BufferDiff>,
         main_buffer: Entity<language::Buffer>,
-        diff_change_range: Range<usize>,
+        diff_change_range: Option<Range<usize>>,
         cx: &mut Context<Self>,
     ) {
         self.sync_mut(cx);
@@ -2510,6 +2520,10 @@ impl MultiBuffer {
             .diffs
             .insert_or_replace(base_text_buffer_id, new_diff);
 
+        let Some(diff_change_range) = diff_change_range else {
+            return;
+        };
+
         let excerpt_edits = snapshot.excerpt_edits_for_diff_change(buffer_state, diff_change_range);
         let edits = Self::sync_diff_transforms(
             &mut snapshot,
@@ -2705,7 +2719,12 @@ impl MultiBuffer {
         let base_text_buffer_id = snapshot.remote_id();
         let diff_change_range = 0..snapshot.len();
         self.snapshot.get_mut().has_inverted_diff = true;
-        self.inverted_buffer_diff_changed(diff.clone(), main_buffer.clone(), diff_change_range, cx);
+        self.inverted_buffer_diff_changed(
+            diff.clone(),
+            main_buffer.clone(),
+            Some(diff_change_range),
+            cx,
+        );
         self.diffs.insert(
             base_text_buffer_id,
             DiffState::new_inverted(diff, main_buffer, cx),