git: Side-by-side diff UI (#47349)

This PR implements a UI for the side-by-side diff, using blocks to align
the two sides and adding a coherent `SplitEditorElement`.

Release Notes:

- N/A

---------

Co-authored-by: cameron <cameron.studdstreet@gmail.com>
Co-authored-by: Anthony Eid <hello@anthonyeid.me>
Co-authored-by: Zed Zippy <234243425+zed-zippy[bot]@users.noreply.github.com>

crates/git_ui/src/commit_view.rs

@@ -884,7 +884,7 @@ async fn build_buffer_diff(
             diff.update_diff(
                 buffer.text.clone(),
                 old_text.map(|old_text| Arc::from(old_text.as_str())),
-                true,
+                Some(true),
                 language.clone(),
                 cx,
             )