Improve #filter_out_boxes_fully_contained_in_others

`sorted_boxes` was first sorted, then used in an `#any?` check. In worst-case
time complexity, this is just as (in)efficient as checking against the
unsorted boxes.

We can do better by sorting boxes by beginning ascending and ending
descending. Algorithm explanation in comments.

Author: jas14

lib/super_diff/core/tiered_lines_elider.rb

@@ -153,15 +153,22 @@ def box_groups_at_decreasing_indentation_levels_within(pane)
       end
 
       def filter_out_boxes_fully_contained_in_others(boxes)
-        sorted_boxes =
-          boxes.sort_by do |box|
-            [box.indentation_level, box.range.begin, box.range.end]
-          end
-
-        boxes.reject do |box2|
-          sorted_boxes.any? do |box1|
-            !box1.equal?(box2) && box1.fully_contains?(box2)
-          end
+        # First, sorts boxes by beginning ascending, range descending. (Boxes may
+        # never share beginnings, so the latter may be useless, but this is at least
+        # sufficient if unnecessary.)
+        #
+        # Then, iterate through each box, keeping track of the farthest "end" of any
+        # box seen so far. If the current box we are on ends before (or on) that farthest
+        # end, we know there is some box earlier in the sequence that begins <= this one
+        # (because of the prior sorting), and ends >= this one; that is, the current box
+        # is fully contained, and we can filter it out.
+        sorted = boxes.sort_by { |box| [box.range.begin, -box.range.end] }
+        max_end = -1
+
+        sorted.reject do |box|
+          contained = box.range.end <= max_end
+          max_end = box.range.end if box.range.end > max_end
+          contained
         end
       end