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/git_ui/src/text_diff_view.rs

@@ -49,7 +49,7 @@ impl TextDiffView {
         let selection_data = source_editor.update(cx, |editor, cx| {
             let multibuffer = editor.buffer().read(cx);
             let source_buffer = multibuffer.as_singleton()?;
-            let selections = editor.selections.all::<Point>(cx);
+            let selections = editor.selections.all::<Point>(&editor.display_snapshot(cx));
             let buffer_snapshot = source_buffer.read(cx);
             let first_selection = selections.first()?;
             let max_point = buffer_snapshot.max_point();