commit view: Fix layout shift while loading commit (#44548)

Fixes a few cases where the commit view would layout shift as the diff
loaded. This was caused by:
- Adding the commit message buffer after all the diff files
- Using the gutter dimensions from the last frame for the avatar spacing

Release Notes:

- commit view: Fix layout shift while loading commit

---------

Co-authored-by: MrSubidubi <dev@bahn.sh>

crates/editor/src/test/editor_test_context.rs

@@ -283,8 +283,7 @@ impl EditorTestContext {
                 .head();
             let pixel_position = editor.pixel_position_of_newest_cursor.unwrap();
             let line_height = editor
-                .style()
-                .unwrap()
+                .style(cx)
                 .text
                 .line_height_in_pixels(window.rem_size());
             let snapshot = editor.snapshot(window, cx);