Fix clippy errors.

src/parser/headline.rs

@@ -77,9 +77,9 @@ fn _heading<'b, 'g, 'r, 's>(
         // If the section has a planning then the timestamp values are copied to the heading.
         if let DocumentElement::Section(inner_section) = &section {
             if let Some(Element::Planning(planning)) = inner_section.children.first() {
-                scheduled = planning.scheduled.clone();
-                deadline = planning.deadline.clone();
-                closed = planning.closed.clone();
+                scheduled.clone_from(&planning.scheduled);
+                deadline.clone_from(&planning.deadline);
+                closed.clone_from(&planning.closed);
             }
         }
         children.insert(0, section);