Add diffing of quote greater blocks.

src/compare/diff.rs

@@ -370,6 +370,11 @@ fn compare_greater_block<'s>(
                 );
             }
         }
+        "quote" => {
+            if first_child != "quote-block" {
+                return Err("Quote greater blocks should correspond to a quote-block cell.".into());
+            }
+        }
         _ => todo!(),
     }
     let mut child_status = Vec::new();