Reduce display_map snapshot creation (#39354)

Re-applies https://github.com/zed-industries/zed/pull/30840

This PR re-applies the initial
[PR](https://github.com/zed-industries/zed/pull/30840). As it was closed
because it was hard to land, because of the many conflicts. This PR
re-applies the changes for it.

In several cases we were creating multiple display_map
snapshots within the same root-level function call.
Creating a display_map snapshot is quite slow, and in some
cases we were creating the snapshot multiple times.

Release Notes:

- N/A

crates/editor/src/test/editor_test_context.rs

@@ -265,7 +265,10 @@ impl EditorTestContext {
 
     pub fn pixel_position_for(&mut self, display_point: DisplayPoint) -> Point<Pixels> {
         self.update_editor(|editor, window, cx| {
-            let newest_point = editor.selections.newest_display(cx).head();
+            let newest_point = editor
+                .selections
+                .newest_display(&editor.display_snapshot(cx))
+                .head();
             let pixel_position = editor.pixel_position_of_newest_cursor.unwrap();
             let line_height = editor
                 .style()
@@ -590,7 +593,7 @@ impl EditorTestContext {
     fn editor_selections(&mut self) -> Vec<Range<usize>> {
         self.editor
             .update(&mut self.cx, |editor, cx| {
-                editor.selections.all::<usize>(cx)
+                editor.selections.all::<usize>(&editor.display_snapshot(cx))
             })
             .into_iter()
             .map(|s| {
@@ -688,9 +691,12 @@ pub fn assert_state_with_diff(
     expected_diff_text: &str,
 ) {
     let (snapshot, selections) = editor.update_in(cx, |editor, window, cx| {
+        let snapshot = editor.snapshot(window, cx);
         (
-            editor.snapshot(window, cx).buffer_snapshot().clone(),
-            editor.selections.ranges::<usize>(cx),
+            snapshot.buffer_snapshot().clone(),
+            editor
+                .selections
+                .ranges::<usize>(&snapshot.display_snapshot),
         )
     });