Preallocate vector for MultiBuffer::excerpt_ids (#49381)

This saves some time spent reallocating inside `Editor::on_buffer_event`
for large multibuffers.

Release Notes:

- N/A

crates/multi_buffer/src/multi_buffer.rs

@@ -799,6 +799,7 @@ pub struct ExcerptSummary {
     excerpt_locator: Locator,
     widest_line_number: u32,
     text: MBTextSummary,
+    count: usize,
 }
 
 #[derive(Debug, Clone)]
@@ -2046,12 +2047,10 @@ impl MultiBuffer {
     }
 
     pub fn excerpt_ids(&self) -> Vec<ExcerptId> {
-        self.snapshot
-            .borrow()
-            .excerpts
-            .iter()
-            .map(|entry| entry.id)
-            .collect()
+        let snapshot = self.snapshot.borrow();
+        let mut ids = Vec::with_capacity(snapshot.excerpts.summary().count);
+        ids.extend(snapshot.excerpts.iter().map(|entry| entry.id));
+        ids
     }
 
     pub fn excerpt_containing(
@@ -7746,6 +7745,7 @@ impl sum_tree::Item for Excerpt {
             excerpt_locator: self.locator.clone(),
             widest_line_number: self.max_buffer_row,
             text: text.into(),
+            count: 1,
         }
     }
 }
@@ -7840,6 +7840,7 @@ impl sum_tree::ContextLessSummary for ExcerptSummary {
         self.excerpt_locator = summary.excerpt_locator.clone();
         self.text += summary.text;
         self.widest_line_number = cmp::max(self.widest_line_number, summary.widest_line_number);
+        self.count += summary.count;
     }
 }