Diff view (#32922)

Todo:

* [x] Open diffed files as regular buffers
* [x] Update diff when buffers change
* [x] Show diffed filenames in the tab title
* [x] Investigate why syntax highlighting isn't reliably handled for old
text
* [x] remove unstage/restore buttons

Release Notes:

- Adds `zed --diff A B` to show the diff between the two files

---------

Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>
Co-authored-by: Ben Brandt <benjamin.j.brandt@gmail.com>
Co-authored-by: Agus Zubiaga <agus@zed.dev>

crates/git_ui/src/git_ui.rs

@@ -22,6 +22,7 @@ mod commit_modal;
 pub mod commit_tooltip;
 mod commit_view;
 mod conflict_view;
+pub mod diff_view;
 pub mod git_panel;
 mod git_panel_settings;
 pub mod onboarding;