editor: Save base buffers when applying changes from their branches (#19562)

This PR makes it so that when we apply changes within a branch
buffer—currently just the edits buffer—we save the underlying buffer.

This also fixes an issue where new files created via edits were not
properly flushed to disk.

Release Notes:

- N/A

---------

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

crates/editor/src/proposed_changes_editor.rs

@@ -298,6 +298,20 @@ impl Item for ProposedChangesEditor {
             Item::set_nav_history(editor, nav_history, cx)
         });
     }
+
+    fn can_save(&self, cx: &AppContext) -> bool {
+        self.editor.read(cx).can_save(cx)
+    }
+
+    fn save(
+        &mut self,
+        format: bool,
+        project: Model<Project>,
+        cx: &mut ViewContext<Self>,
+    ) -> Task<gpui::Result<()>> {
+        self.editor
+            .update(cx, |editor, cx| Item::save(editor, format, project, cx))
+    }
 }
 
 impl ProposedChangesEditorToolbar {
@@ -323,7 +337,7 @@ impl Render for ProposedChangesEditorToolbar {
             if let Some(editor) = &editor {
                 editor.update(cx, |editor, cx| {
                     editor.editor.update(cx, |editor, cx| {
-                        editor.apply_all_changes(cx);
+                        editor.apply_all_diff_hunks(cx);
                     })
                 });
             }