Clarify which of the candidates is the right one - removing assert

lib/tree_builder.py

@@ -118,8 +118,9 @@ class TreeBuilder:
                 candidates = []
             node = node.parent
         if candidates:
-            # the assert is here to see if we have more hot cases
-            assert len(candidates) == 1
+            # the first candidate is the youngest one that matches the check. That's
+            # good enough. See FastCGI test case for rev 36 and 38: 37 reverted 36 and
+            # then 38 reverting the revert before it was submitted.
             return candidates[0]
 
     def add_merge_points(self, factory_revisions):