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/language/src/buffer.rs

@@ -4268,6 +4268,11 @@ impl BufferSnapshot {
         self.non_text_state_update_count
     }
 
+    /// An integer version that changes when the buffer's syntax changes.
+    pub fn syntax_update_count(&self) -> usize {
+        self.syntax.update_count()
+    }
+
     /// Returns a snapshot of underlying file.
     pub fn file(&self) -> Option<&Arc<dyn File>> {
         self.file.as_ref()