diff: also sort base ranges by end point

I wanted to replace the BTreeMap by a Vec and noticed that we actually
sometimes end up having a `0..n` range followed by a `0..0` after
refinement. We currently compare those two as equal because I had not
thought that we could end up attempting to add two ranges with the
same start point. When trying to insert the second range (`0..0`), the
BTreeMap will keep the existing key (`0..n`) and replace the
value. That's probably works, but it's clearly not what I
intended. Let's fix by sorting by the end point if the start point is
equal. This actually improves some benchmarks by a few percent (maybe
because the subsequent compaction can then remove the `0..0` range).

lib/src/diff.rs

@@ -297,13 +297,13 @@ struct BaseRange(Range<usize>);
 
 impl PartialOrd for BaseRange {
     fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
-        self.0.start.partial_cmp(&other.0.start)
+        Some(self.cmp(other))
     }
 }
 
 impl Ord for BaseRange {
     fn cmp(&self, other: &Self) -> Ordering {
-        self.0.start.cmp(&other.0.start)
+        self.0.start.cmp(&other.0.start).then_with(|| self.0.end.cmp(&other.0.end))
     }
 }