git: Use a buffer for the panel's commit message (#23308)

This PR changes the `GitPanel` and `GitState` to use a
`language::Buffer` for the commit message. This is a small initial step
toward remote editing and collaboration support.

Release Notes:

- N/A

---------

Co-authored-by: Max <max@zed.dev>

crates/project/src/project.rs

@@ -691,7 +691,7 @@ impl Project {
                 )
             });
 
-            let git_state = Some(cx.new_model(|cx| GitState::new(cx)));
+            let git_state = Some(cx.new_model(|cx| GitState::new(languages.clone(), cx)));
 
             cx.subscribe(&lsp_store, Self::on_lsp_store_event).detach();