commit view: Display message within editor (#44024)

#42441 moved the commit message out of the multi-buffer editor into its
own header element which looks nicer, but unfortunately can make the
view become unusable when the commit message is too long since it
doesn't scroll with the diff.

This PR maintains the metadata in its own element, but moves the commit
message back to the editor so the user can scroll past it. This does
mean that we lose markdown rendering for now, but we think this is a
good solution for the moment.


https://github.com/user-attachments/assets/d67cf22e-1a79-451a-932a-cdc8a65e43de

Release Notes:

- N/A

---------

Co-authored-by: cameron <cameron.studdstreet@gmail.com>

crates/editor/src/editor.rs

@@ -22598,6 +22598,10 @@ impl Editor {
         }
     }
 
+    pub fn last_gutter_dimensions(&self) -> &GutterDimensions {
+        &self.gutter_dimensions
+    }
+
     pub fn wait_for_diff_to_load(&self) -> Option<Shared<Task<()>>> {
         self.load_diff_task.clone()
     }