migrator: Fix case where users see migration banner despite no diff changes (#26982)

Fixes edge case where after carrying out all migrations if final text is
same as existing text, we don't need to ask user to do anything, despite
migrations edits are being applied internally. E.g. A -> B - > C -> A

Release Notes:

- N/A

crates/migrator/src/migrator.rs

@@ -77,7 +77,7 @@ fn run_migrations(
             result = Some(migrated_text);
         }
     }
-    Ok(result)
+    Ok(result.filter(|new_text| text != new_text))
 }
 
 pub fn migrate_keymap(text: &str) -> Result<Option<String>> {